Nothing like ruined mounts to eat up a tranny. Now that the B2134 :-) is installed and (mostly) tuned, it's time to take care of some other nagging concerns.

Under heavy load (0-60 in how many seconds?) in first or second gear, and under moderate load in third or fourth gear, and at high rpm in third or fourth gear, there is a rattling sound coming from my transmission, that sounds kind of like a metal ring rattling around a metal rod.

Any idea which part(s) I need to order?

Since you mentioned that the noise occurs in all forward gears, specifically 4th which is straight through, you can pretty much rule out any of the pesky little needle bearings. That leaves main input shaft bearing, and output shaft bearing.

Input shaft bearing is pretty robust. Have never had one fail. I have seen one output shaft bearing fail, and somehow managed to replace that without pulling the tranny. Just shake the driveshaft and see if there's play.

Or as others have said, quite possibly something is simply loose. And that would be a more likely thing to happen with excess vibration from bad mount... Definitely check them things first!

And, of course, fluid level.

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-Matt I ♥ my ♂

On my transmission the rattle was coming from the shift lever. The shift base under the cap sits in a grove/socket. This grove gets worn over time and allows the shift handle to rattle. Unfortunately, the only remedy was to find another transmission cover with an unworn socket. My transmission is now quiet!

I appreciate your advice, everyone. I will check those spots and report back on the findings.

When I say B2134 I am referring to the B20 that I had bored.

Check the rear seal - that popped out on my last M40, and it was making some pretty interesting noises in all gears.

Not sure exactly what a B2134 is, but is the cap that holds the shifter handle into the tranny screwed down tight?
I've run a bunch of M40 and M41 trannies and had lots of bad mounts but have not seen tranny damage relatable to that unless the tranny was completely flopping loose. In fact looseness at the FRONT of the tranny is a lot more serious.
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George Downs Bartlesville, Oklahoma

I would check the shifter cap first.

If not, check for a loose flange nut (the big one) on back of the tranny.

Paul