strange clacking noise, any ideas?

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I just bought a 82 240d w/ a 300d turbo engine. It has a strange clacking noise that I cant seem to pinpoint. The noise increases with RPM but seems to go away at high RPMs (this car does not have a tachometer so I am not sure on exact numbers). I used a mechanics stethoscope and probed all over the engine (injectors, vac pump, IP, oil pan) and couldn't find anything. It sounds like it is coming from the lower half of the engine somewhere. When I stick my head under the drivers side it sounds like the noise is coming from the front sump area. When I listen under the passenger side it sounds like the noise is coming from the transmission. I loosened each injector line and the noise did not change. When I drive the car I can make the noise completely disappear by applying strong pressure to the brakes. If I come to a hard stop and stay firm on the brakes after the car has stopped the noise will completely disappear. The instant you release the brakes the noise appears again. This is driving me crazy. Anyone have any ideas? Anything else I should look for?
thanks

The only engine component that would have anything to do with the brakes is the vacuum pump. I do not know why applying the brakes (using vacuum) would stop the noise, but I can't think of any other possibilities.

Originally Posted by Craig

I do not know why applying the brakes (using vacuum) would stop the noise, but I can't think of any other possibilities.

Because it shifts how much of the engine/tranny weight is on each mount. Releasing the brakes relieves the tension (flex) that the brakes lock in the suspension and body as you come to a stop.

Check for loose bolts, loose engine accessories and worn/broken engine mounts.