Hey fellas. I have a loud noise from the rear of the car, and I'm leaning toward the differential as the cause.

Here's the story. Purchased the car from a VW dealer after doing my own inspection. The only issue I found was oil splashed on the rear right axle. They provided paperwork when I asked about it showing they had replaced that seal, so I assumed they just hadn't cleaned up the mess. Car made no noticeable noises during the test drive or the 3hr drive home. Car was bought for track use, so I didn't drive it much before installing an AP Racing brake kit and four new OEM c5Z wheels with Toyo R888R tires in a square setup. After installing that stuff, we took it for a test drive and heard very loud whining from the rear of the car. The pitch of the noise changes depending on speed, but the noise is constant. It doesn't change with throttle load or turns.

I got home and checked under the car and noticed that axle seal was definitely still leaking, as there was oil dripping from the axle boot, electrical connections in he area, and running down the exhaust. Drained the oil from the diff and saw that it sparkled, but nothing was magnetic. Filled it full of fresh fluid and took it to the dealer, who spent a few days trying to figure out where the noise was coming from. He eventually hooked up a chassis listening device and said he was 80% sure it was coming from the diff, but wouldn't commit to an absolute decision.

So, I drained the original diff fluid into containers and measured the fluid, and it was down at most .04-.05qts. Definitely not enough to matter.

So that's where I'm at folks, and I have no idea what's truly causing the noise, or why the diff suddenly started making noise if that's the case. It was like flipping a light switch. No noise all the way home, installed the parts...suddenly making noise. Brake kit was only installed on the front.

Any ideas on how I can prove it's a wheel bearing or the diff without taking the whole drivetrain apart to inspect it?