another differential whine...

i recently changed from 2.73 to 3.42 gears in the rear end of my 80 malibu. we changed the carrier and ring and pinion with new bearings. we left the car on jacks and ran it for 2 minutes in drive and 2 minutes in reverse, then we let her down and drove about 10 miles at different speeds. there was no whine, no humming, no noises at all. on the way home from the shop though, about 20 minutes into the drive... the differential started to hum. at first it was just higher speeds... 30 mph+, but now it makes noise at pretty much all speeds. sounds like a hum on acceleration and more of a whine on deceleration. i'm wondering why it slowly started making the noise, but was fine for the first 45 minutes or so. i'm guessing it is because it would have been too easy to make the noise with the mechanic right there, i mean then it could have been fixed and that just makes no sense, ha ha. seriously though... any thoughts? thanks.

oh, when i finally got to my house after babying the car the whole way... something smelled funny from the differential housing, i touched the cover and felt it was super hot.

Was any grease put in it. Have it towed back to the mechanic so he can repair his mistake.

2 quarts of gear oil were put back in. the mechanis has the car back and is fixing it. i wrote this post while at work, before he had the car back... just to see if anyone had any ideas, or maybe something that i could look for myself. i am very new to the whole differential part of cars and new to these forums as well.