Wheel came off and rotor landed on the cement as I was driving the other day. Noticed a noise after replacing.?

So apparently it seems that the bolts weren't tight enough on one of my wheels. I was going about 30-35 mph when the whole thing suddenly came off and landed on the disc/rotor. Still shocked, I noticed that the gear lever (while still in drive) seemed to be stuttering. Anyways, I shut the car off, mounted the spare and took it home. After taking a look, it seemed that the disc and rotor got slightly bent. Also two of the lug nut holes had a good beating and opened up a bit. So I went to the junk yard and had the whole unit replaced. Now, everything seems perfectly normal.. except now I'm noticing a weird humming/almost like a tin can scraping noise happening when I'm driving slow. Almost seems that it happens when its about to change gears (especially at around 2,000-2100 rpm). So now I'm worried that something had happened to the tranny. My uncle, who's a mechanic, says that its probably the differential and that I should be fine. But I just want to see what anyone else thinks. By the way, I drive a '96 Automatic VW Jetta.