Just replaced headgaskets on 97 deville concours. After car warms up, there is a very light knocking noise coming from the rear of the engine compartment at idle through 1000 rpm, then it goes away. Car is running premium gas and has no valve clatter, even during heavy acceleration. Heads only had .003 shaved off of them at the machine shop. Car runs SUPERB otherwise. Car did not make noise before gasket replacement. I read on other post that this could be the #1 bearing thumping caused by serpentine belt lifting against damper. I hope this is it at least. Maybe its time for marvels mystery oil and a for sale sign?