Hi all,

I have a 2007 z51 m6 that has recently developed a humming noise in the rear end I am having trouble diagnosing. Sounds like tire roar but still present while car is up off the ground. The noise is ground speed dependent, no change with rpm or taking car out of gear while moving. First thought was wheel bearing but no play in either assembly, and when I ran the car up on jack stands, I still couldn't tell a clear difference between one side and the other, though the jimm was definately present.
About a month ago I swapped my Michelin ps2's for re-71's. Immediately I noticed increased road noise though it did not sound out of the ordinary for such an agerssive tire. Ran the car on SCCA Targa:southland didn't notice any dramatic change through the event however, when I put the Michelin back on, the humming was as loud as ever! I figure it developed gradually while I had the other tires in but I can't say for sure. Also, when I was changing to the re-71's the first time, I also exchanged the diff fluid. Was getting the "gravel" sound in the diff in parking lots, replaced with gm recommended stuff, no more diff noise! However, as this was a change point I can't help but wonder if the noise is coming from the diff...
I've tried running the car while up in the air, definately coming from drive train but I can't pinpoint which side! Can anyone help me with a better way to diagnose this issue before I have to start disassembling the rear end?

 

I know that sound, my 07 with 47k has had it for a while. Took it to shop turns out left rear bearing and hub. I'm not sure how they figured out which side it was though. It was driving me nuts too, some days louder then seemed to go away. I tried reading hubs with heat gun after a drive in 100deg day, both were within a couple deg. It gets replaced in the morning so I can ask how they found it. I suspect it was with a stethoscope while running on the lift.
Good luck

 

Thanks Gambler, yes please ask them how they found it.

I tried the stethoscope, I'm just not 100% sure what im supposed to be hearing. Both rear wheel bearings sounded equally terrible compared to the front so I think that is the issue, just not 100% sure since it seems unlikely both would be bad. Also listened to the diff, kind of loud metal and fluid moving sound but no different than the trans, and since the noise is present even when out of gear, I'm confident the trans isnt the issue, so rear end I think is OK.
I ordered 2 rear wheel bearing assemblies yesterday, going to dig into them this weekend. Hoping off the car, I can tell a distinct difference between the good and bad bearings.

 

Sounds like a plan. I was on the way to the shop and they called and said they could not get to it until Monday. Sorry I couldn't be more help.

 

Update, replaced both rear wheel bearings this weekend. No more hum! Niehter bearing had any play in it and I was skeptical that both had gone but once they were off the car, it was pretty clear they were the source of the noise. Both sounded like they were full of gravel and the replacements were much quieter.