Tom,
From the TDIFAQ.
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Rattles, buzzes, and resonances from the exhaust system:
- Many B4 (1995 to 1997) Passat models have a resonance in the exhaust system at 1200 rpm, and a lesser one at 2400 rpm. Several possible causes have been described but the problem appears to be inherent in the design.
- Some owners have reported that the downpipe (flex pipe) has become clogged with carbon build-up, thus causing noise to be transmitted through the rest of the exhaust system. In some cases this component has been replaced under warranty. In other cases, the problem was addressed by removing the flex pipe from the car and working it back and forth to loosen up the deposits.
- Some have had one of the mufflers (there are two) come apart internally, thus requiring replacement.
- Exhaust hangers can harden up and/or work loose over time, thus transmitting much more noise to the passenger compartment. These components are easily replaced.
- If this happens out of warranty, some owners may consider this to be an excuse to buy a performance aftermarket "cat-back" exhaust system. This might be the only way to eliminate the flaws in the standard system. The TDI engine is not loud even when operating with no muffler at all.
- Several owners have had success with getting a muffler shop to install an additional flex pipe downstream of the catalyst.
- Deteriorated engine mounts can increase the amount of noise and vibration transmitted into the vehicle.
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Jerry