73 280 funny hiss/rattle, from trans?

[wolf_walker]

It sounds rather like a rattle snake, is the only way I can think to describe it.
Only in gear and moving, is throttle sensitive but I think it's load and/or RPM based. It tends to build till a shift occurs then start over, so I'm assuming it's the trans. I believe it's not present in 4th, but it may be drowned out by road and engine/wind noise. If I drive a speed such that it stays in 3rd I can vary the intensity of the noise with small throttle changes, it's more prominant with heavier throttle.

Fluid and filter are fresh, trans shift soft on 2-3 at light throttle, firm from 1-2 and 3-4, 3 sec delay into reverse. All in all pretty typical for an old Mercedes. Also the bushing on the trans side of the control linkage is MIA, so that might account for some of the shifting oddness. I've driven a lot worse examples here and there. It shift earlier than i think it should, but again there is slop in that linkage till I can create a bushing of some sort.

Anyway, anyone heard this noise before? Is it fixable or just a sign of a tired trans, and I should put up with it till it gives up? I never heard this perticular sound from the 80's models I'm more familier with.

Thanks...

[mercmad6.3]

Check under the car for the exhaust bracket at the rear of the trans. Also there are sheet metal heat shelds above the exhaust and covering the center bearing mount.