What is the little plastic cap-like piece on top of the rear end housing? I was getting a wierd noise from the rear of my car at around 2400 rpm (in all gears). The noise subsides as I speed up and goes away at ~3000 rpm. So I jacked the car up and noticed what looks like rear end diff oil all over everything underneath the car. I noticed the little cap piece so I wiggled it and it came off. Could this noise be a result of a rear end going south? Because the little cap was loose, could all of the rear diff fluid leaked out? It's an '02 M6.

 

That little "piece" is the ABS sensor. There should be a small bolt (takes a 10mm wrench I think? ) that holds the sensor in place. If you were able to wiggle it out without pulling a bolt, then obviously the bolt is gone, and it could have loosened on its own, and is probably the source of the diff oil leak. Well, that sounds reasonable anyways!

So what you need to do is put that sensor back, and get a new bolt to hold it in place. Then if you've lost fluid, you'll need to re-fill the diff, so get yourself a bottle of fluid, get the car jacked up and there's a plug on the passenger's side of the diff housing that takes a 3/8" ratchet to remove, pull the plug and fill via that hole until you get fluid starting to pour back out, then you know it's full. Good luck with it!

 

Could also be the little plastic cap on top of the vent tube.

 

It's common for the pinion seal to leak; this could be the source of the drool.