Chevrolet Repair: 1994 GM 3.1 Engine Noise, 1994 pontiac sunbird, pontiac sunbird

Question
I have a 1994 Pontiac Sunbird (3.1, automatic, 109k) that has recently been making a loud "knock" type sound from what seems to be the top of the engine.  I've been working on cars for years, and have never encountered this one.  It is not a rhythmic type knock (relative to engine rpm, and almost subsides when the rpm's are raised slightly), it's more erratic, and prominent at idle.  I can "feel" the noise through the upper intake plenum if I rest my hand on it, but can't tell exactly what it could be coming from.  With each knock the otherwise fine running engine, coughs for a millisecond, then fully recovers until it happens again, usually 2-3 seconds later.  It's a little deeper sounding than a typical valvetrain tick that I would imagine, but I don't have a great deal of experience with the 3.1.  I know the car has never had a timing chain done, so that could be part of the problem, as the noise tends to dissipate somewhat at speed when the engine is "tightened up", but I can't say for sure.  
I hope this description is sufficient to give you an idea of the problem we're experiencing and I look forward to any information you may be able to provide.  Thanks very much.

Sincerely,

Dave Turner
New Jersey


Answer
As you know from working on cars. I would need to personally HEAR this sound in order to make any suggestions as to what it could possibly be.

I don't recall any other complaints nor have I run acroos any 3.1's making this type of sound.

Without hearing it myself, Anything that I could suggest would only be a guess and I am a LOUSY GUESSER.


I wish I had the answer for you but I just do not.


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