I know it's almost impossible and could be a million and one things, but I thought I'd see if this case rang any bells with anyone.
87 745t M46 trans.

When cold, between gears as the RPM's fall I get a rattle, the sound like something get's going through a resonance as the rpm's fall.
It will only do it a few times before it's gone, or occasionally after it's sat for a spell. My first "oh crap" thought was the CBV making a bad noise when throttle lifted, but near as I can tell it's fine. The way it's temp dependent I guess eshaust is the most likely thing, but how the heck does one track something that will not manifest sitting still, and only occasionally when rolling.
It's pretty darn annoying, old car sounds like a tractor when it's cold as it is.
Thoughts?