On my '87 5k, I have a problem, probably with the valves. When the car is idling, I can feel/hear a "puff" out from the tailpipe, a lot stronger than the normal exhaust pulses. This disappears if I rev the engine a bit, but wil re-appear after half a second at idle. I know that the engine is supposed to have hydraulic lifters (new some months ago) so I don't really know what's wrong.. :-( Any suggestions? ES