venture99

I have a 2000 z28 M6 with 101k miles on it and ever since I bought the car a year ago, it has made a pretty loud rattling noise between 4-5k rpm at WOT when the engine reaches operating temperature. I was pretty sure that it was detonation, but doesn't detonation usually occur at lower rpms? I have also heard it before at lower rpms too.

I have already replace a faulty pcv valve which helped it run smoother since there was a massive vacuum leak, but the noise continued. I then tried mopar combustion chamber cleaner, but it didn't smoke one bit and the noise continued. I then replaced my worn serpentine belt, and this made the noise noticeably quieter. It's still there, but it isn't nearly as bad as before. What's weird is that I put on a brand new belt, but it still squeaks sometimes when the engine is cold. This made me think it could be the belt tensioner. Should I try replacing that or would it be a waste of money? What about the A/C belt?

Mercier

iTrader: (15)

I suggest you run it very briefly with no belts; this will eliminate your accessories. Keep in mind you likely have two belts; one smaller for the A/C. Is the noise different in/out of gear? Clutch engaged/disengaged? How is oil pressure when the noise is there vs. isn't and cold vs. hot idle? You could even have a chunk of catalytic converter bouncing around. Need to get as specific as possible with where the noise is coming from, and exactly the conditions under which it presents. You could even take a video. Some keen ears around here.

Spark knock/detonation, to me, always sounds more like a tinkling in the cylinders if that makes sense..a true rattle I'd say sounds like something failing. But detonation can cause things to fail; in extreme cases blow holes in pistons and all kind of fun stuff like that. Detonation can happen anywhere RPM-wise but usually under a load. Like WOT at low RPM or even at high RPM. Too much advance, compression, load, and not enough octane. Even stock PCM programming is going to increase ignition timing as RPMs increase(Ok; it's not quite as simple as a distributor but I'm talking simply here) so you can have detonation any time things aren't just right.

That being said, you have two knock sensors, that if working, should cause the PCM to retard timing almost immediately in the presence of knock and reduce or eliminate said knock and give you a nice momentary power decrease.

I suggest you try the easy stuff (accessories/belt) first because as you go down the list, fixes get far more spendy..like bearings.

Good luck!