I've heard belts make noises that would convince you that there was metal knocking.

There's also the possibility that your hearing the sound of early detonation. Air leaks in the fuel system perhaps? Make sure none of the little fuel return hoses at the injectors are wet. Or any of the fuel hoses for that matter.

Edited to add:
That air filter bracket you guys mention is almost always broken. So that's probably it.

I stopped having that problem when I replaced the engine dampers.

__________________
Aaron,
Albuquerque
1983 300 SD
1976 300 D