Humming Rear Driver Side Only Around 70mph

[optiontrader]

109K, all stock.

Had similar issue on the opposite rear passenger side around 60K (yep, right after warranty expires), where it would start humming around 50mph. Thought tires - needed replacement anyways, but didn't fix. Dealer said bearing going bad - replaced; humming noise goes away.

Fast forward to now @109K, I've got a similar humming noise @70mph, but on the driver side. So I go “I know what this is” and go to replace the wheel hub assembly. However, while I'm underneath, I notice the black gunk on the diff. I don't think anything of it at the time - I finish replacing the wheel hub, but unlike the earlier problem on the passenger side - the humming is still there @70mph.

So I'm now thinking, the black gunk is from the axle, it's definitely on the drivers side rear so I purchase a replacement (reman) axle to slap on.

But searching thru the forum, the black gunk is most likely coming from the differential bushing.

So my question (axle's off right now): could the diff bushing cause this humming, and only around 70mph? There are no clunks or other noises that I can hear.

Insight, oh 370Z gurus?

[optiontrader]

Bumping; got some news.

The reman axle is longer than the existing one. Looking online from Parts Geek where I purchased it from (it's a Surtrack, NI-8358), it shows the driver side longer than the passenger side. Can't be - the diff protrudes more onto the driver's side - that half axle has to be shorter. So I'm going to return the axle.

So do I give Z1 a call on their diff bushing set (gonna put the car back together in the interim)? As the black gunk isn't coming from the axle - the axle should be good.

Again - will replacing the diff bushings (I'm gonna be looking for a used set of Z1's tool) solve this noise?

[Rusty]

The black gunk is silicone from the diff bushing running down the diff. Your bushing is ruptured.

As for the humming. Could be a couple of things that could cause it.

[optiontrader]

Thanks for the reply. Do you think the humming could be caused by the diff bushing rupture? As I mentioned, I replaced the driver's side rear wheel hub assembly, and was about to replace the axle.

Seems I should return the axle (as it doesn't fit), order the diff bushing set (and removal tool - I'm not taking a saw out), replace - and see if the noise goes away.

Is there anything else you think I should check/test out?

[Rusty]

The bad diff bushing is not making the humming sound. Check your rear tires for wear and a wear pattern. Wear patterns will cause different types of noises as the tire wears. How many miles on the tires? Air pressure in them? Inside worn more then the outside? Tread blocks have a feathered edge? Lets eliminate tires all together.

[SouthArk370Z]

I'm with Rusty - check the tire(s) first. There's a pretty good chance it's the problem; it's cheap and easy to check. If you don't have the know-how, take it to a reputable tire shop for inspection.

[optiontrader]

You know what - I completely discounted the tires. When I heard the similar noise @60K, it remained after I replaced my tires and only went away after the dealer replaced the wheel hub. I incorrectly went in on this issue thinking it had to be the wheel hub/power train - and not even considering the tires.

So it seems I've got a smoking gun here: tires are Yoko ADVANS - 9/4mm F/R tread left noted on last oil change a couple of weeks ago. But they're more then three years worn, and due for replacement. Snapped some pics - I put one below (PBucket ain't letting me link more than one) - it's obvious wear measurements were only taken on the outside (left side of pic):



Not easily seen in the pics, but as expected, the surface of the tire curves noticeably inward towards the right edge of the pic. Would this be a good suspect on the cause of the noise?

I'll put the Z back together and start shopping; not only for the diff bushing stuff, but also a new set of shoes.

[optiontrader]

Here's the profile, edge-on pic:

https://imgur.com/a/HbPtW

[Rusty]

4mm is .157 or 5/32". The rear is close. Is there any high and low worn spots in the edge of the tire?

[optiontrader]

None that I can see. The wear is pretty smooth; as can be seen in the above imgur post, profile is pretty straight until to get to the right most radial tread groove, where the tire wears noticeably downward towards the inside face of the tire.

But I'd think that would be expected given the negative camber of the rears.

Just made a tire appointment to replace the rears this Sat; I'll let everyone know whether that solves this issue.

[optiontrader]

Rep to Rusty and SouthArk370Z - the tires were the noisemakers. Just replaced with a new set of Yoko Advan All-Seasons on the rears, and the noise is gone. Thanks!

I took a look at the diff bushing while reinstalling my left rear axle, and its still intact, save for a small (1/2") tear on the front side which allowed the filler material to leak out. May still have a good number of miles to go, but I'll keep an eye on it.

Thanks again!!!