Engine? noise in Audi 80

["Mauricio A. Leon" <mleon@sun1.ece.missouri.edu>]

I have an Audi 80, 1992 with just over 36K and, to my surprise and
disappointment, the engine? started to make a noise that is similar to a
strong valve noise.  This clicking sound is more audible when the engine
is cool, is not always related to the engine RPM, and is more notorious
after the gas pedal is suddenly released.

I will profoundly thank any and all opinions concerning the origin,
test, and solution to this -expensive sounding- noise.

p.s. have not noticed any engine performance change, and always used 91
or better gas.