1998 Mustang GT rear end noise
 

I just got done replacing a rear pinion seal today on my car in my auto class. I had the help from the teacher. We took everything apart, and put it all back, once we got back to the rear flange, we realized it had a lot of play in it. Once we torqued the pinion nut to the specific torque, it still had play in it. So my teacher deceids to grab an impact gun and tighten it that way even though in the servie manual it says NOT to use air tools. Once everything was bolted together, i started to drive it and now when i slow down is has a loud "hum" noise to it. I was thinking maybe its the bearing rubbing against the pinion or something? If someone could help me out, I'd really appreciate it, I really need to fix this.

RE: 1998 Mustang GT rear end noise
 

This is the cheap way of getting out of a pinion seal replacement! The reason there was a little bit of play is because your crush collr had already been torqued down to spec once and they are really only meant to be torqued down once although garages get away with changing the pinon seal as you did without any problems. My guess is when he grabbed the impact gun, he over torqued the crush collar and that changes the backlash! You'll have to tear apart the whole rear end this time to changhe your cruch collar! Or maybe it's still not tight enough?

RE: 1998 Mustang GT rear end noise
 

Yeah, I was talking to some Ford Mechs. and they said it most likely isnt tight enough and said to take the new one out and replace it with the old one and tighten that down. So, I'll have to hope for the best!