used gears= noise?

a friend of mine recently installed used 4.56s in his rear 44.(still running 3.07s up front, but keeps it in 2 for now) i dont know the numbers offhand, but he said they set them to the fsm. he has a decent amount of sound coming from the back. very slight on accelleration and pretty noticeable on decelleration. i seem to remember reading on here that this is pretty common with used gears, but after about an hour of searching i didnt come up with much. i told him it was the probably the used gears, and if i could get any reassurances that would be great.

most likely. i put used 4.88s in my D44 front, but with hubs it doesn't spin on the road, and i never use 4wd over about 20mph, so I didn't worry about it.