Buzz or chatter sound at IP?

[jkubica]

Greetings All

My 1985 300td seems to be running fine. But I notice a buzing or chattering sound that seems to come from the area of the injector pump when I'm under the hood and rev up the engine. It doesn't make the sound at idle or at higher RPMs - but only at a certain small RPM range.

Does anyone have an idea what would cause this sound?

Thanks
Joseph