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Hi! I need help in diagnosing a rear-end hum/growl, and in deciding if it's worth repairing the truck. Bottom line: my shop believes I need a new rear differential. Total cost including labor and tax: $2,400. (Factory-new part).



Here's the story.

I have a 2005 Suzuki XL7 4WD with ABS. 137,000 miles (220,000km). Just put on *brand new*, "top-shelf" Goodyear Assurance Comfortread touring all-season tires. (They're awesome, btw...highly recommended). They have less than 5,000 miles on them. So I'm confident the noise is not the tires.

Bought the truck in '09 with 30,000 miles and have taken great care of it. I love the truck.

Edmunds.com lists the current private-party value at $2,400 (average condition). Hence, my dilemma in putting in a part that costs more than the truck!

In March, I had my local shop change the fluid in the rear differential. I do not know if the prior owner had the 7,500-mile rear diff fluid exchange done (my research on this forum has told me this early exchange is vital to the long-term health of the rear diff). I had it done for the first time since 2009 and was the only time it was done since I have had the truck.

In late May, I noticed a new hum & growling sound from the rear end. I initially thought it was a wheel bearing. Brought it back to my shop. They were not convinced it was a wheel bearing. At the time the sound was still very faint, so I continued to drive. No repairs at that time.

I added around 3,000 miles and the sound got worse. I took it back to the shop and they did a test drive. They also lifted it and used a stethoscope to isolate the noise to the rear diff. They pulled the drain plug and took a sample of the "new" fluid for a cursory search for shavings. Nothing noted. (The magnet was not checked as that's apparently labor intensive to get to it).

WHAT IT SOUNDS LIKE...

The sound changes pitch, not in volume, based on speed. It's a humming-growl that it first noticeable at around 25mph. It gets higher in pitch the faster I go (pitch...not volume). When hovering around 35-40mph, under constant steady load on the accelerator, the growl is constant. If I let off the gas and coast, the growl is basically the same but wobbles just a bit as the driveshaft is in a coasting phase. I did a bit of slalom-turns at a medium turn at around 35-40mph and my non-expert ear suggests there is not an increase in the noise under a weight-bearing turn in either direction. Seems to suggest that it's not a bearing also.

QUESTIONS:
1) Is this consistent with a rear differential that is in need of replacing or repairing?
2) If this is the rear diff, is it worth it with a car of this age?
3) Is $2,400 an appropriate cost for replacing it with a factory-new diff?

BY THE WAY:
-The car still has its original timing chain and exhaust components.
-The car has some issues with the AC components. Long story there for another time, but matters when taken in context with the issue at hand. Blows cold and dries properly, but AC light flashes and intermittently does not work.
-I have never towed with the truck. BUT...I *just* installed a class-3 hitch and was planning on hauling small cargo trailers at times for home projects.
-My shop thoroughly inspected the car for any rust concerns along the underbody. Rust is normal for our area and does not appear to cause concern about not repairing the car due to rust.
-Due to New York regulations, it is apparently not advisable to purchase a part on your own for a shop to install. Local shops will install only a factory-new part, or will rebuild the part the customer has based on its ability to be repaired.

THANKS EVERYONE!
I'll do my best to answer follow-up questions as they arise. See you on the internets!

Rick Bancroft
Rochester, New York, USA
 

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Rear end noise can come from several different places - the wheel bearings, which you seem to have eliminated, the differential bearings (there are four - two on the pinion, and two that support the hemisphere) the diff gears themselves. and finally the ring & pinion.

In my experience, ring & pinion gears will be noisy primarily in one direction - on drive - or - on coast - but not in both (and please note that when you change direction - forward/reverse - drive/coast will also change), and they tend to howl more than growl.

The diff gears themselves rarely give trouble, and so I suspect your problem (if it is the rear end) is likely to be the bearings, and those are replaceable - however - it takes skill & equipment to do a proper job, and those, of course, come at a price - I won't discuss pricing because, you & I are in different countries, but I will say that I can get bearings replaced at a price that makes it worth my while to replace them.

See if you can get hold of a "chassis ear" (or a shop that has one) - it's a set of microphones that can be attached to various components and then you drive the vehicle and select the microphones one by one to see which gives the loudest (or best) pickup - make sure to listen to the drive shaft support bearing.

If you're not hearing the timing chains rattling on a cold start - I would not worry too much about them, and the a/c problem is most likely a door binding or sticking.
 
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