My 85 does this after I screwed with the idle adjustment screw on the IP. Once the car is hot and I go into park, I can hear the valve clatter increase as well as a rise in oil pressure. Tach doesn't register it but I don't trust that for accurately measuring engine RPMs.
I lowered the idle a bit since Bill's GTG. I had it set too high and pressing the "stop" switch wouldn't kill the engine.
For a high mileage engine like mine, I'm thinking of keeping it at that high of an idle. I know it masks symptoms, so if you want to send me a brand new timing chain and valve seals please do
I want to also replace my rack dampener eventually. I have a nice vibration at idle even with new mounts.