I picked up another Jeep a few weeks ago. Its got an SM420 mated to a D-300.
When I let off the throttle (in gear) or when I'm compression braking going down a steep hill it sound like there's a tin can full of rocks rolling around in the tranny. Its fine out of gear & when accelerating.
I suspect a bearing is dying somewhere, or is the noise just inherent to the old 420's.
BTW: all the d-shaft u-joints are good.

Just thought I'd ask before tearing it apart.

-Dan

mine rattles to an extent, and thats normal.

I rebuilt my sm420 and it makes noise when not in gear because the shaft (2 piece) is slightly worn. Kind of a wa-wa-wa sound. It's been bomb proof, but sounds rough, like a HD truck tranny. Go figure?

Could be the syncros are worn.

Yeah, mine too. Gets worse over 2500rpm for me. I found once my Dana 300 shifter was quite loose and that made it sound worse.

David