Hello all, I have a 1984 Z28 with a stock L69, save for deleting most of the emissions equipment that normally makes the car impossible to work on IMO. The car had a rough life before I got it about 2 years ago. I had to immediately replace the head gaskets, because (according to my detective intuition) the fan switch went out and the PO decided to ignore the thermometer. I've driven this car pretty sparsely because I have been fighting vacuum leaks and other tic tac problems. Finally it runs without dying, though rough.

It has always made this noise. I really hope it isn't rod knock. It isn't audible from inside the car with the windows up, and you can only hear it while driving when the window is down and the sound bounces off of a parked car or building. It doesn't increase with revs (or perhaps the engine drowns it out when above 1500 RPM). I have pawed around with the exhaust and other hard lines, and nothing seems to be rattling around underneath. Last oil change was clean (no sparkles), though I know most of the metal can end up in the filter, which I don't have the equipment to cut open. I should be clear, I have decent mechanical experience (enough to change these head gaskets at least) but I am VERY green to muscle cars and V8s, so my biggest hope is that this is just normal noises of a 305 in this vehicle, though I am probably wrong.

Any help or expertise is appreciated, and here is the video. Thank you for reading!

First thing I would do is pull the valve covers and check the valvetrain.

Sounds like bottom end, to me.

If you have an auto trans, (I didn't see a dipstick for one), you could check converter nuts and for a cracked flex plate.

It is an automatic. How would I check this? Do I need to drop the transmission?

Take the tin inspection cover off of the front/bottom of the bell housing, then you can see the flex plate and nuts. You'll have to rotate it to see all of it.

I am going to inspect that flex plate today or tomorrow depending on the weather. Been reading threads about it and it seems like the trans has to be dropped to change it unless you're proficient with wood blocks and a jack lol. I've also read that it's hard to see the likely location of the crack (commonly by the crank bolts). We shall see... I will update with some pictures when I get more information

Yep, trans has to come out, if the plate is cracked.

That sounds like a rod knock, to me, but a cracked plate can make the same sound and is way easier to inspect than dropping the oil pan. Another diag trick you could do is kill cylinders and listen for a marked change in the sound of the knock. If one cylinder markedly changes the sound, when it's cut off, then that is pretty strong evidence of a bad rod bearing on that cyl.

Hard to diagnose over the internet; but when you check the flex plate, check for exhaust leaks too..

when my flexplate bolts got loose on the crank it allowed the flexplate to walk back and forth, sounded exactly like this, the fix was new flexplate bolts, your flexplate might have cracked too. I had to drop my tranny and put new bolts in, I think I used the spectre ones because the ARP ones don't allow for enough room to get a socket on it since the crank flange is right next to it


Sounded almost EXACTLY like the sound in your video when the engine was running, its pretty easy to check too. It almost sounded like something was loose, getting dragged around in the bell housing

jaimie, do you have an update?

Not hard to change out the flex plate without dropping the transmission.

Support the transmission in a way that will allow it to slide rearward: floor jack oriented front to rear w piece of plywood under the pan, actual transmission jack, etc. Remove the converter bolts, torque arm & crossmember. Undo the cooler lines from the transmission. Pull out 2 of the bell housing bolts, 1 on each side. Put about 6" long bolts in their places. Remove the rest of the BH bolts. Slide it back as far as it will conveniently go. Use a long box-end wrench on the flex plate bolts.