JMon

I get knocking at low RPMs when I press on the accelerator a little. Not WOT.
I don't know if it's an O2 sensor problem, MAF or something else that's affecting the timing. Doesn't happen all the time.. Just every once in a while.. Doesn't seem to be linked to getting a new tank of gas. Any ideas?

hightest

It could be the carbon buld-up issue. Try the GM engine combustion chamber cleaner, and see if it works.

SpinMonster

Did you auto-tap the car or are you just going by sound? How much Knock retard is present? If it is just engine load in 5th and 6th just pull timing there. Warmer weather affects the fuel curves