My G/F has a '91 RS with a 3.1 MPI. Lately it has developed a low knock, sometimes not audible but sometimes loud enough you can hear it at idle at a light inside the car. It does not seem to be lifter or valvetrain related, the oil pressure is at least 25 hot at idle and 60 psi cruising down the road. It could be rod, main or piston slap. Anyone else ever have this?
Can the oilpan be removed with the engine in the car? I want to check this out ASAP.
The car runs great and the knock does not get louder with increased RPM and load that I can hear. DOn't know the history as far as miles, abuse etc but it looks clean inside the oil fill cap and does not smoke or use oil.

 

I have a similar noise in my 97 lumina with the 3.1.
I bought the car with 19k miles on it with the noise and I have 60k on it now. I took it to the dealer because I bought the extended warranty on it and he told me the noise is common in the 3.1.
It sounds kinda like a lifter noise but it the source seems to come from the center of the engine and not up top.
I have drove the car 40k miles like it, it seems to be harmless, I hope.

 

Hey the knock is probably caused by the Knock sensor on your car. Mine had the problem, and as soon as i replaced the sensor it stopped. The code light wont always fire to code 43 when the dang thing goes bad.