Odd knocking sound
 

Hey guys, after my engine has reached full temp, if I close the doors and windows and lean towards the dash, I can hear a subtle knocking sound. This knocking sound has existed for near two years now, and has not gotten any worse. For this reason, I don't think it's any form of rod-knock. Once past idle, it either goes away or can't be heard any more. The more and more I think about it, the more I think it's a misfire. It can be heard if you stick your head near the engine in the engine bay as well (driver side). At idle, the car runs a little rough, but no-where near a full misfire. Its more or less just a tiny bit shaky. It also runs on the rich side. When hot, the injectors will make a sporadic, very loud click. This click makes no distinct pattern, but not every injector does it. Injectors all ohm out to 16-17ohm, so I don't believe it's the drivers. Outside of this, there are no problems with it. Nice and smooth throughout the RPM range, no loss in power.

Engine only has 70-80,000 miles on it, oil has been changed religiously with synthetic. It has overheated once while I owned it. I replaced the O2 sensor recently with a new AC delco unit, the compression checked out on all cylinders last time I did a compression test. Ignition system replaced including plugs, wires, cap/rotor, and control module. When datalogging, the computer shows and constant rich condition, except when decelerating. All of this points me more and more toward a misfire, but would a misfire make this sound? It's not valve tick, it much lower in pitch. I'd like to blame it on injectors, but I want someone elses opinion before I drop $300-400 for a new set.

Re: Odd knocking sound
 

Wondering if your odd knocking sound could be a lifter that is leaking down? Have you checked for this? Sounds similar to a condition I had in a 71 Camaro I once had. Turned out to be a lifter that wasn't holding prime.

Re: Odd knocking sound
 

Anyone else? I'd like a couple opinions before I tear into the engine. After reading about heavy lifter leak down, you might be quite on the money jtsk.

Also, there is a very noticeable misfire while accelerating, most noticeable in 3rd and 4th gear. This seems to disappear above the higher rpm ranges however, and full throttle. I can feel the misfire in the pedal and hear it in my exhaust. The misfire noise is coming from the exhaust and not the engine, so I don't think it is rod knock.

Re: Odd knocking sound
 

If it's a low tone, I think I have the same thing. It's usually every other rotation and it sounds like air pumping. Now (since yesterday) my car is difficult to start and stumbles at idle until it warms up, and my dad thinks one of my lifters are bad or out of adjustment. I think I've had a blown head gasket for a while now and it's finally taking it's toll. I am not sure man. I have overheated the car and had a sudden pop with smoke coming from the driver's side.. It might be my problem. But I have noisy lifters with high oil pressure.

Re: Odd knocking sound
 

My first thought would be piston slap. Lifters have a very unique sound.