I'm getting a very loud tensioner clatter when I'm at a stop sign with my foot on the brake. I've tried to pinpoint the noise with a stethoscope, and it seems to be coming from underneath the tensioner wheel. When I park the car with the parking brake on, and run it in Drive or Reverse, the clatter begins. It's louder by far than the engine. I can observe the serpentine belt vibrating wildly and perhaps slapping the pulleys. When I put the car in Neutral or Park, the clatter goes away immediately, and the serpentine belt runs smoothly around the pulleys. I'm suspicious of the alternator clutch pulley (though it feels okay) and the tensioner itself. What else in the belt drive area could make such a racket? It happens ONLY at low RPM, with the engine loaded. By the way, I just put in a new dual mass flywheel, which has helped the shudder a lot and removed some of the general noise, but did not affect this tensioner clatter, which is getting worse.