Hi,

Oh boy I sure do wish you luck in the oil level being the cause!

Since I know so little about the M90 or the 900 series as a matter of fact I went out to see if I could get an illustration of one.

https://forums.turbobricks.com/showthread.php?t=94325

I ran into that above and found it very interesting about the type of clutch used to drive the M90.
It's Interesting, in that, it's a different design than any Chevy or 240's
If you read enough of the articles on this site you will get what I mean about being different.
There is a discussion about the worthiness of it all too!

After reading it I will stick with my original idea that the "hollow" sounds with the hint of "ringing" is not a counter shaft bearing running dry on oil sound.
A main driveshaft support bearing yes, but then, when their dry it's more of a high pitched shrill. Same for a tensioner bearing on the engines timing belt.

On most all manual gears boxes the reversing lay shaft, or a countershaft to some folks, is found in the bottom of a gear box.
It is always submerged in oil and out of the way so to speak because it use so infrequently.
That is except in the motion pictures (Fast &Furious) where they go as fast in one reverse gear, as others go in all their forward gears!
Anyway,
This is so the main throughput gears are up high to keep the driving output shaft up higher in the driveline design.
The oil tracks itself up to the top of the gear case by the way of the gear teeth and splashing about for the most part.
If a A pump was use, this will add expense and can be "parasitic" draw on the final output power.
This went on for years and worked against the acceptance of automatic transmissions.
So, consequently you don't find many manuals provided with powered lubrication.

Today, that has all changed and now look out, here comes the electric motors with variable everything! (::--)

Good luck and make sure you get the right oil for it!

Phil