This is a long post, and I kinda ramble a bit since I've edited it several times already....
I bought a used FM built motor (1999, wiseco, carillo, fidanza, fererra) a couple years ago. Took off the pan before I installed it, the oil looked good, and I didn't see or feel any signs of bearing issues as far as one can tell without removing the caps. Since the first time I started it up, it made an irregular 'loose' clattering sound that I initially thought was valvetrain (that kind of tappity sound). Previous owner made 500+ HP on E85. When I asked him about the noise, he said it always did that and thought it was either from the lightweight flywheel or piston slap, he put about 6K on the motor. Initially I was worried that I had bought a hurt motor, but I've got about 2000mi on it myself running 15psi and 7500RPM shifts. The noise comes and goes, but never got any worse. It usually doesn't make the noise for the first 5 seconds of a cold startup. It's at it's worst at idle, and goes away as revs increase. It seems to get better after driving the car hard, but not all the time. The knock sensor doesn't seem to pick it up, and I can only maybe hear it just a little through a stethescope on the block when I rev the motor up to 3K. I don't hear it at all while the motor is accelerating, and maybe just a bit at a steady 3K, then it gets noisy as the engine decelrates. It almost sounds like it's coming from outside the block rather than inside. When I release the throttle and the revs drop it makes a loose 'tappity tap tap tappity tap kinda sound' I now believe it's the oil pump, but I can't figure out what would make it rattle.
Here's what I've tried/checked:
1. There were a couple valve shims that were a bit out of spec but I fixed them anyway. No change.
2. Removed all accessory belts, this seemed to make it louder, or at least more regular.
3. Quadruple checked all the timing belt stuff to make sure the pulley's weren't moving and everything was torqued down properly.
4. Modified the factory oil pressure gauge and installed an 80psi sender in the factory location. Cold idle oil pressure is about 70psi, hot idle pressure is ~40psi. Cold driving pressure is 80+psi, hot driving pressure is about 60psi. I'm guessing a little since the gauge isn't linear. I run Rotella 5w-40 oil. Plenty of oil is getting to the head.
5. Noise is the same with the clutch engaged/disengaged.
6. Changed out the crank pulley.
7. Disassembled the last two oil filters looking for any metallic stuff, didn't find anything.
8. Ran the **** out of it hoping that whatever it was would get worse or break so I could stop worrying about it.
Here's my reasoning for the oil pump:
If I take the metal rod off the mechanics stethescope and place the open tube near the crank pulley, I can hear the tapping better than with the metal rod on the block! BUT the weird thing is that with the rod ON the stethescope, I may be crazy, but then it seems louder on the exhaust side of the head. I thought maybe the sound was from something on the exhaust cam and that the plastic timing cover was sorta ducting the sound down to the crank pully (biggest opening in the cover) but I can't think of anything else to check up there.
The noise miraculously went away for a whole afternoon right before my last oil change, when I changed the oil it came back full force.
But what the hell is there in the oil pump that would rattle??????
If the gears were broken I would expect more drama. Something wrong with the relief valve? Maybe the super-lightweight flywheel is making the crank rattle it as it drives the pump gear?
I had Ron Williams of RAW Performance (very respected Subaru engine builder that has built a few BP engines) look at it, and he said if the oil pressure was good, he wouldn't worry about it, but he didn't know what it could be. Had an e-mail discussion with Jeremy at FM. Didn't get much info besides try some heavier oil, and sometimes they are a little noisy. I understand that remote diagnosis is difficult though.
I've attached a best effort recording of the noise. this was at a 750ish RPM idle through the stethescope on the head. Too much 'air' noise to get a good recording through the tube to the crank pulley, but sounds similar.