904 Downshift Squall or Howl

My son's '65 'Cuda has an occasional but subdued howling or squalling sound when it downshifts when slowing to a stop. This occurs are both the 3-2 and 2-1 downshifts. The tone is kind of like the low pitched squalling sound that an 18 wheeler's tire makes when it locks up, but much less loud or harsh and lasts for < 1 second.

The car has a 8.75 rear axle and 3.55 Sure Grip. But I don't think that would squall when stopping in a straight line. We have looked for rubbing of the driveshaft but see nothing like that.

So is there anything in the 904 like a band adjustment or rear overrunning clutch problem that could sound like this?

It also has a Gear Vendors OD unit, but I'll call them directly and ask about that. I want to ping the Torqueflite experts here on any possible 904 issues or anomolies.