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1987 944 Turbo vibration-induced noise at 3200 rpm

Engine

Vehicle Information: Model(924/944/968): 944 Turbo; Year: 1987; Body Type: Coupe; Total Mileage: 94,000;

I have a noise that sounds like buzzing sheet metal (or perhaps muffler internals vibrating) that occurs only at 3200-3400 rpm and only under load. Unfortunately, this noise is 100% repeatable and in the middle of what I believe to be the peak of the engine's torque curve--a nice place to cruise.Quick inspections by technicians at both a local Porsche dealership and local Porsche performance shop have revealed nothing out of the ordinary in the engine compartment or under the car. Routine maintenance is up-to-date and the engine is operating within specs.The previous owner (I'm the 2nd) indicated that the noise might be from the engine guard/shield, but the muffler heat shield also looks like it's about the right size/shape to produce the kind of noise I'm hearing--and I think that's about where it's coming from.Question(s): 1. Do you have any experience with an exhaust-related or torque-induced vibration-related noise on otherwise smooth/quiet-running cars? The engine is smooth and sounds great above and below the resonant rpm range.2. If I am going to try to hunt this vibration down myself, can you recommend a safe way to temporarily damp the vibrations of the muffler heat shield and/or the exhaust pipe while I drive the car on public roads at highway speeds? (As I write this I'm thinking about a wheel balancing weight clamped to the muffler heat shield to change the resonant frequency of that part.)Thanks.Mike

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