I have a 2005 Grand Vitara with about 47,000 miles on it. Lately I've been hearing a noise coming from the rear end. It almost sounds like someone blowing into a bottle. The humm only occurs above 40 miles an hour and is louder when you have your foot on the gas. Does anyone have any idea what would be making this noise!!!

I'm going to guess the rear diff - running with little or no lubricant - and if I'm right, it's already too far gone, and will need to be replaced.  Adding oil will quiet it some, but it's time to replace the ring & pinion.

Yep, probalby rear diff, but first give your exhaust a thorough going-over.  Make sure nothing's touching it, no holes, etc.

Well A noise in the rear really dont help much in terms of what the problem could be.....beercheck and Fordem ideas could be the problem....it could also be unbalanced tires.......heck I have even heard my fuel pump on my daytona make a humming noise........great thing is you can check most of these problems..... so lets do Process of elimination

Fordem's Suggestion......Remove the rear differential plug and dip your pinky in if ya cant feel any oil then your low also check around gasket area for leaks

Beerchecks suggestion - checking for holes feeling for exhaust leaks......or simple while in park idle your vehicle at around 40mph revs and see if it makes that noise while standing still....dont do it to much though

my suggestion Tires -  is easily spotted by looking over your tires for uneven wear which could cause a humming noise.


If you have any more info to add would make figuring out the problem much easier

I checked the rear diff fluid level and it was a little low. While checking it I noticed that the gear oil was really black and decided to change it. I noticed a little difference. I think today I might change the front diff gear oil as well since i was at it. I looked for any holes in the exhaust and didn't see anything. I think I might also get the alignment checked and see if that may be the problem too. Thanks for all of the ideas. Let me know if you think of anything else.

Black OIl.....its supposed to be relatively CLEAR that means it has not been changed in A LONG time  be prepared for the worse to come ...... IF you JUST bought this vehicle I suggest an overhaul of ALL the fluids......(of course not everyone has the money nowadays to do that) but its easy stuff to learn

Black OIl.....its supposed to be relatively CLEAR that means it has not been changed in A LONG time

I'm gonna guess 47,000 miles.  Meh. 

It's probably still the break-in fluid but, on it's own, that's not a deal breaker.  If the (low/contaminated) fluid is the problem, changing it may forestall any serious problem escalation for tens of thousands of miles.

Sounds like a rear U-joint, check ALL of them for movement... clunk when ya let clutch out?

Bryce

I checked the rear diff fluid level and it was a little low. While checking it I noticed that the gear oil was really black and decided to change it. I noticed a little difference. I think today I might change the front diff gear oil as well since i was at it. I looked for any holes in the exhaust and didn't see anything. I think I might also get the alignment checked and see if that may be the problem too. Thanks for all of the ideas. Let me know if you think of anything else.

What did the drain plug magnet look like?  Just a little furry or lots of stuff stuck to it?

The plug magnet had alot of sludge on it but not alot of metal shavings in it. I had check the ujoints but there is no movement in the driveshaft and I haven't heard a clunk but my zuki is an automatic. I replaced front and rear diff fluids. but the noise is still there. Like I said it is only noticable at speeds over 45 or 50 mph. Any other ideas???

Wheel bearings?  Technically speaking they're not in the wheels on this vehicle, but actually in the rear axle, but they do exactly the same job.

Next time you're hearing the noise try a gentle swerve left and right (of course only when it is safe to do so), does the noise change any?  The left/right swerve causes a weight shift which changes the loading on the bearings and if that is where the noise is coming from, the noise will also change.

wind noise from a roof rack? bad door seal perhaps?