HUMMING NOISE

Finaly got the barracuda out for a test run to the local cruis in. I noticed a humming noise when i down shift it into 2nd or 3rd & comming to a stop. Its just a quick humming nois that sounds like its comming from the trany. It doesnt do it all the time. The 833 trany is rebuilt & the 8 3/4 rear has new ring & pinion plus all new bearings. Someone told me it might be the rear end & the noise is transfering up the drive shaft. Could this be ?