valve noise (?)

[zep@galactose.mc.duke.edu (Zvi Pasman)]

Hello all:
I have an 86 Quantum 5, 2.2L.  Recently, the engine has developed a strange
clicking noise.  This appears only after a highway run.  Upon startup,
I get the usual clattering and that gets quiet once a steady idle is
achieved.  After the car is driven about 20 highway miles, the clicking
noise appears when the car is idling (like, when getting off the highway
and standing at a light).  The clicking disappears when the car is
driven, but only after getting to 3rd gear and up, sometimes by 2nd gear
if I wind it sufficiently long at 3500 RPM.  This clicking disappears 
if I then go back on the highway.
The car has 135K miles, burns no oil, passed emissions inspection recently.
Does this sound like an oil pressure problem, or maybe a valve is
sticking?
Any thoughts or comments will be appreciated.

Zvi.