Have you checked cam timing? That will give you an indication of the health of your chain. The IP timing spec (24 BTDC of whatever) depends on proper cam timing as well.

In the US, there is an unusually high incidence of timing chains breaking on 300D 2.5 models. At least it seems high to me. Check the archives for yourself.

That sound can't be on the intake side because there's no air throttle and the intake manifold isn't pressurized at idle.

Another thing you can check, though I don't know how, is the health of your vacuum pump. It's on the front of the engine ahead of the injection pump.

Sixto
91 300SE
87 300SDL
83 300SD