Noise when backing up on a hill

Hello there, so I was backing up a pretty steep hill the other day and I noticed that when I was backing up, some noise came from somewhere. I suspect it is the driveshaft since my uncle's 98' Manual does that too but on a more even grade? (don't know how you'd say it) I recently did the u-joints and carrier bearing as well. Any ideas what it may be and if it needs to be fixed soon?

Edit: So it not just on steep hills, it seems to be just on a normal/average hill. It doesn't seem to happen on flat roads though.