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1991 C2 Hollow Ringing Sound?

Engine

Vehicle Information: Year: 1991; Body Type: Coupe; Total Mileage: 49000;

My engine makes what I can best describe as a fairly faint, sometimes not-so-faint, ringing sound above the music of an otherwise strong motor. The noise is a sort of hollow ringing sound, as if a relatively light weight metal ring 4-6" in diameter were vibrating within a thin walled pipe.My mechanic thought that it might be one of the engine's belts nearing its service life of all things, but he didn't seem supremely confident of that diagnosis. Here's some background...the car was recently bought a month ago, has no service records until my own, but has had one PPI and one post-PI by two different Porsche shops...both of which, after their comprehensive inspections, proclaimed the car to be one of the nicer 964's they've tested. Since then, I've had the valves adjusted, spark plugs replaced, distributor vent kit installed, and a host of other reconditioning items performed. The sound is most noticeable right after start-up when the engine is cold. It changes with RPM, but not necessarily directly so. It either dissappears once under way, or is drowned out by engine and wind noise...I can't tell. When idling at a traffic light, the sound is less pronounced to even non-existent (I'm assuming due to a warm engine). At these times, it drives me slightly batty, in the same way as when you think you hear your mobile phone ringing and discover that sometimes it's not. Before I go down a potentially expensive quest for the elusive sound, does this "ring" a bell for you? Does my description sound like it actually could be one of the belts? What would you recommend as a troubleshooting strategy, maybe starting with a technique for localizing the sound more concretely? Thanks for any insight that you can provide.

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