tranny issue

[1991GSXAWD]

Hi, new here to DSM tunners,
ever since i bought my car its has had a high pitch whine where it gets really annoying at 70mph so i have to slow down to about 60-65 where its not so bad, was wondering if anyone knows where i should be looking into, the noise seems to go away if i accelerate even at 70mph but if i let off the gas pedal it quicky comes back, it has a slight whine in 3, and then it starts to get a little worse when upshifting to a higher gear, any help as where to look for the problem would be very helpful, as i'll be taking her apart hopefully wednesday, also i've changed all the fluids when i bought the car last year, so far i've only put 600 miles on it, i'm running Synchromesh in tranny and Mobil1 in transfercase, thanks in advance,Mike.
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[Old Mitsu Tech]

If it's whining and howling, this will be followed by jumping out of fourth gear and soon after that usually 3rd. This is pretty common in the Manual transmissions. You need to replace all bearings and synchros. Look at the gears very close. Look at the mainshaft bearings also.

Good Luck