It started after I cut the AIR tube off of my cat. The cat had a rattling sound. The heat shield on the top was loose, so I put a hose clamp around it until I can get an exhaust shop to replace it. No difference. The sound only happens around idle speeds. As the engine speed increases towards 1,000 RPM, the sound goes away. Load has no effect on the sound. I tried decelerating with the engine, and I didn't get the classic rod knock sound there. It can mainly be heard when inside the car. Once you get outside and try to hear it, it's barely audible. If you listen under the car, it's the same volume as how it sounds in the engine bay...barely audible. Any tips?

 

Please, any tips? I looked at what I could see on the flex plate. All of the bolts holding on the torque converter are tight. The sound can barely be heard outside of the car. It's loud as hell inside of the cabin. Listening to it outside from above or below, it still sounds internal and barely audible. It does it when hot or cold. It does it without any belts attached. My oil pressure is just fine. It's its loudest when in gear and not moving. At a high idle it's a little difficult to hear, and the sound disappears by 1,000 RPM. It runs just fine. I recently got rid of my Edelbrock water pump and got a regular one, regular flow. Temps are getting a little higher now, but not within overheating point. It only got warm enough once to turn on a fan (235° or 238°), and that was during a time when this sound has been occurring.

 

do you have one of these?
http://www.harborfreight.com/mechani...ope-69913.html

it might help you isolate the noise?

 

Nope. I was planning on getting one on Friday, if the car is still running by then.

 

sometimes they're pretty helpful. you can also try a long screwdriver if you dont have a stethascope.

 

When you cut the air line to the cat, did you cap the tube on the cat?

 

It's not like I'd have a clue of what to be listening for, or where to listen. What kind of internal engine noise would be loud inside the cabin but barely audible above and underneath the engine? The mounts all seem good, but I don't know how to go about checking that without actually removing them. A cracked flex plate seemed like a good idea, but IMO that would be very audible outside, and even more audible with the inspection cover removed. I don't think I would need to pull the transmission back (no way I'm can do that) to verify that kind of problem.

Yes.

 

Wouldn't be surprised if it's busted-up catalyst banging around inside the cat.

 

I guess I will find out after I gut the thing. It did rattle a lot when I sawed off the AIR tube and banged in the plug, but I discovered the heat shield on top was barely attached...

 

It was not the catalyst. It didn't surprise me that it wasn't. Why would such a thing be loud inside of the cabin and sound like it's on the other side of the firewall and be barely heard outside of the car. I really have no desire, nor time and flat, solid ground, to separate the trans from the engine to check the flex plate.

 

Sure it isn't coming from inside the cabin?

 

What is there inside of the cabin that would have a metal-on-metal type loud knock that hits the same speed as the crank? I haven't actually recorded the sound and compared it against a datalogged tachometer reading to be all scientific and absolutely sure, but I'm fairly certain that it happens somewhere along every rotation of the crank. It can be heard outside, but very faintly, and it sounds like it's in the engine. I don't get why something inside of the engine would be louder in the cabin than outside.

 

It was something to consider.

The cabin represents something of a hollow "drum" and has its own natural resonance. It's possible that the low frequency knock is close in frequency to the natural resonance frequency of the cabin. That's why it sounds amplified in the cabin. Anything vibrating close to or at its natural resonanice frequency will be at or near max amplitude.

 

I think if it was a case of resonance the sound would seem like it would be coming from everywhere instead of sounding like it's on the other side of the firewall. I do know what you're talking about. With some cars with hard motor mounts and the windows cracked, at the right RPM it sounds like there is a subwoofer inside.

 

Depends on what is exciting the resonance. But yes, with low resonance as in a subwoofer, it sounds like it could be coming from anywhere.

All I'm saying is that the reason it might sound louder inside the cabin instead of in the engine bay is that the cabin is amplifying whatever is happening on the other side of the firewall.

 

Exhaust rattles are usually louder in the car. Check along the length of the exhaust pipes for contact with the body or crossmember. You can also get under the car and hit the exhaust with the palm of your hand or a rubber mallet and listen for rattles. Used to do that every time we run new pipes on a car. Did you remove the air pipe completely or just cut it? May be rattling against the exhaust pipe or body. Check the rubber isolators on the exhaust mount brackets.

 

Nothing on the exhaust is contacting the body. The pipes have been completely removed. The one to the cat and the one that runs along the back of the engine. That one was tough to get and I had to use quite a bit of force to pull it out. The pipe that goes to the heater core was a suspect of hitting the firewall. That's been debunked. The speed is consistent with RPMs and it never falters, like something external rattling might do.

 

take a video so we can hear it

 

When pulling that pipe out from behind; could you have hit the distributor cap making the rotor come in contact with the cap? This might sound directly behind the firewall and be hard to hear from the outside.

 

I changed the cap and rotor right after I pulled the pipe. I cut the pipe in two and pulled each part out the side, not through the top. I'll try to record the sound later.

 

You didn't happen to get the cap on crooked did you?

 

Nope. I don't even know how that could be accomplished as far as then getting in the screws that hold the cap to the distributor. I threw on the old cap and rotor just for entertainment, and no difference.

Here is the video.

 

Idk how it would be accomplished either. I have seen some pretty jacked up stuff before though. I watched the vid. I don't have a clue.

 

Anyone?

 

Hard to tell from the video. Is the cabin fan running? Could possibly be that. Or the vacuum that controls the fresh air valve for the hvac could be pulsating. You could try and disconnect that and see if the noise goes away.

Can you hear it from under the car? Sometimes when a rocker is too loose or a lifter is bad you hear it resonate from the bellhousing area under the car....which may sound like it's coming from under the dash.

 

The cabin fan is not running. That has its own noise problem of its own right now (squeal). I can barely hear it from outside the car, both above and below the engine. Which vacuum line is this? There is no such diagram in my factory service manual. I don't even have an option on my HVAC to switch between fresh air and recirc. It seems to give me fresh air all the time.

 

There are vac lines that run from the controls to the engine bay. They are usually solid vac lines that come out of the fire wall. One goes to the heater valve and the other to a vacuum source.

 

I unplugged the plastic line that tees in above the wiper motor. No change. As I said, it goes with the speed of the crank. It's metal-on-metal, and sounds like it's coming from the engine when heard through the firewall.

 

I'd pull the valve covers and check those rocker arms.

 

Check what about them?

 

see if one is overly loose, and then re-set the rockers to zero lash.

 

I thought these things had hydraulics in the lifters to make lash zero.

 

I don't hear a knock in that vid, I do hear the load onto the engine and what sounds like a vacuum or exhaust leak. Maybe try taking a vid from the outside, or cut a piece of tubing and try and locate the source that way.

 

I didn't post up a video when I opened the thread because I don't believe anything but professional equipment is going to be able to capture how it really sounds. It sounds like something contacting something else, not a leak of any sort. I still have no idea what I would do with a tube or stethoscope. The sound just can't be heard outside. If someone wants to come diagnose this, be my guest.

 

You'd be surprised what some leaks can manifest.

Tube/scope will allow you to locate from where the noise is coming from. Stick it in your ear and point around, the noise is directed by the tube/scope so you can pinpoint the origin of the sound. Sounds weird, but works, try it.

 

Yes, very hard to distinguish what that noise is on the video. Perhaps try and take a video under the hood with the engine running.

As far as the rocker adjustment goes. Just because you have hydraulic lifters doesnt mean that the rockers don't need initially set to zero lash. All that means is you have more wiggle room than an engine with solid lifters. Find a youtube video on adjusting rockers on a small block chevy with hydraulic lifters and follow the process. It's pretty easy and it may fix your noise. If not then you can rule out a component in your search for the noise.

 

If a phone video can't pick up the sound that well in the cabin where it's actually loud in reality, it's not going to pick it up in a noisy engine bay where the sound can't even be heard with human ears. I'd rather just have someone come diagnose this than waste time myself I don't have in the first place. I really think it's an unfixable internal noise.

 

OK, here's the scoop on the stethoscope. I probed around all various places. Nothing. Curious to hear what the fuel system sounded like, I put the end of it on my soft fuel line. A very loud knock was heard. Tracing towards the engine the knock gets louder. It's really loud up at the fittings and bracket near the thermostat. If I probe around that immediate area, like on the thermostat housing or the hose clamp for the one heater hose, nothing. Thoughts?

EDIT: The fuel line bracket on the intake manifold was loose. Tightened that up. The fittings weren't that tight. Tightened them up. No change. I'm beginning to think it's purely from these stiff fuel lines (braided stainless teflon) I put on and they're transmitting engine movement to the hard lines on the body and the aged brackets are letting the lines rattle. I'm going to wrap some aluminium around the lines where they're at the brackets and see if that does anything. One thought that bothers me is why it's loudest up at the engine rather than down at the body...

 

If you can get under the car Id try tapping the cat with a soft rubber mallet to see if anything came loose inside. Especially if you used a sawzall to cut off the tube. That could really vibrate the hell out of the cat. I doubt the brackets have much to do with it and are just transmitting the noise from somewhere else.

 

The cat is gutted. I've already established that this noise is in the fuel system.