I have a 1999 Oldsmobile 88 LS. It developed a knock, which started out as a marbly rattly sound. It makes both of the sounds in tandem right now, marbly sound on driver side of the engine, knock on the passenger side toward the bottom, around where the harmonic balancer is. The sound sounds exactly like metal smacking/spanking metal.
I'll tell you what I've tried already due to the massive amount of feedback I've gotten, but nothing has helped me out.
1. Changed harmonic balancer.
2. Changed alternator.
3. Changed oil/oil filter.
4. Changed timing chain/tensioner.
5. Replaced all exhaust gaskets.
6. Changed coolant.
7. Changed spark plugs/wires.
8. Dropped oil pan to check for trashed metal bits; found none.
9. Checked for exhaust leaks, found none.
10. Disconnected belt (no accessory pumps running) and ran the car for about 10-15 seconds. Sound remains.
11. Checked the flexplate. No discernible cracks.
12. Pulled 1,3,5 spark plugs. Spark plug 1 white tip is noticeably darker than 3 and 5.
13. Tried disconnecting spark plug wires from coils as car was idling to see if I could isolate the sound to a cylinder. Sound remains no matter what.
I am at a total loss as to what this noise is. It disappears at 3000RPMs and above, and comes back as soon as you let off the throttle and the RPMs go under 3000. I've been told that although it sounds like rod knock/rod bearing, it can't be, because when I checked the oil pan, I didn't find any real metal bits in there (just some extremely tiny flakes, which look normal), and because ever since that sound started, I've driven well in excess of 1000 miles. If it was a rod bearing or something like that, the engine would have been dead by now. Also, I don't think it'* the flexplate, as the sound is not random. If it were the flexplate, it would be a random noise; this sound is perfectly timed (as in engine moving parts timed), and is perfectly reproducable.
Here is a video of the noise.
YouTube - noisy car
I would appreciate any and all help. I'm on the verge of saying it'* the lifters and/or the pushrods, but I need some feedback first. I'm feeling pretty certain that it'* the lifters, especially after reading this:
Or could it be the knock sensors going bad?
Thanks in advance!