So, like the title says, there is a strange knocking noise coming from the motor when I rev it to around 3500 RPM. It has never done this before and it doesn't do it all the time either. I have considered that the knock could be..... knock, but I dont think my timing would freak out an then fix itself 5 minutes later. <--- maybe my distributor clamp has gotten loose
Could it be the valves rattling around?

EDIT: '86 V6 4-spd

[This message has been edited by MordacP (edited 02-12-2008).]

It could be a rod bearing. If so its a lot of work to fix. Let hope its something else.

How does your timing "freak out"?

Could be piston slap as well. A really easy way to tell would be to blip the throttle, to get the engine to rev to about 4k, then let off. If it is piston slap, you will hear it loudly as the RPM's drop back down. In general, if it is piston slap, it will be much more noticeable when the engine is cold.

-Darryl

Does it happen suddenly?
If so it's a rod bearing
Happened to me last summer
Went into a resto to get a coffee and when i started the car everything was fine until i hit the 3,000 RPM in first then in second and so and so .
Was the number 5 rod bearing i went 30 miles with it .
Opened the pan and was -.020 missing on the crank
Got the engine rebuilded

[This message has been edited by Robert 2 (edited 02-12-2008).]

I revved it up to 4k, let the throttle off, it sounded really cool the way it always does, but no knocking. It hasn't happened for a couple days. I'll just get under it and look for stuff that is loose.

Thanks for the info, i'll keep it in mind when it happens again. If it happens again.

Then it could be a piece of carbon that went off a piston ?

Keep a close eye on that noise. It sounds very similar to what I had. For me, it wasn't a rod bearing, but in fact a main bearing on the block. The motor would run fine, idle fine, everything. However, one day, I started noticing a knocking noise at high rpms. It wasn't a knock so much as a muffled thud. As I drove the car more over the next few days, it developed more and more of a knock. The rpm that the knock would appear at started to show up at lower and lower rpms. Next 3,500 rpms, then 3,000 rpms, and then once it hit those rpms, I started to notice that my oil pressure was dropping too. The warning light would then illuminate when the car was warmed up, and the lifters would rattle (due to lack of pressure). I ended up rebuilding the motor and it turned out to be a couple of my main bearings. The metal actually squeezed out the size of the bearing.

For what it's worth, the car had overheated VERY badly a month or so before. The water pump impellar had broken free from the water pump shaft, and the car immediately overheated.

------------------
Todd,
2006 Pontiac Solstice
2004 Volkswagen Beetle Convertible (Wife's Car)
2002 Ford Crown Victoria LX
1987 Pontiac Fiero SE / V6 (3.2) 5-Speed
1984 Porsche 944 5-Speed
1981 Pontiac TransAm (Olds 455BB)
1973 Volkswagen Type-2 Transporter