Today I test drove a truck I am looking at buying (07' 4x4 quad cab hemi) and noticed a humming coming from the rear end at certain speeds. The hum was noticeable at about 55km/h +- 5km/h. The sound was very sensitive to speed, if it was out of that range it was silent.

So... Question is, what is it? It sounded like typical rear end hum but it did not change whether the truck was at full throttle or if it was just cruising, it seemed to be solely dependent on speed. When I searched people had mentioned the possibility of a rear brake pad dragging. How would I check to see if this was the cause?

I am really hoping it isn't the rear end because the truck is the exact truck I want at the right price. Any help is appreciated.

 

Private sale or dealer?

If its a dealer ask them for 1 year warranty on the truck after having them check it out.

If you do pick it up definately change the fluids and have it inspected.

 

private sale unfortunately so there is no option for a warranty. FWIW he says the fluids were changed this past summer and they did not mention any problems with the diffs when they did it. I am not sure who he had the fluids changed by, dealer or someone else.

 

Does the truck have the factory LSD? These are known to have the retaining clips break off and do damage to the ring & pinion. (Do a search in this section for "C-Clips" and you can read for days).

Also, if it's got aftermarket gears the backlash could be off just a tad, causing the hum.

Bad thing is, you won't know unless you remove the diff cover and I'm sure the guy ain't gonna spring for a diff service just so you can check out the rear end. So you probably need to decide how bad it sounds and if you are prepared for possible rear end work if you buy it.

I know if I bought it, I'd be popping the diff cover off day one (although I've always done ALL fluids within a week or two of buying any used vehicle anyway)...

 

Could also be something as simple as a tire rotation to correct the problem. What type of tires are on the truck?

 

Is this true? What about the aftermarket warranties out there? Any of them worth the paper they are printed on?

 

After market warranties that are getting spammed all over the TV are worth even less than the paper they are printed on.

 

I don't believe the truck has aftermarket gears, pretty sure its just the stockers. As for the c-clip, I did a search and people seem to say that there are no symptoms of broken c-clips, you just find them laying in the fluid when you change it, so there is no sound related to a broken c-clip? I'm assuming then that the sound means that the broken clip has been left too long and damage has already occurred to the ring and pinion which are now making noise?

 

Okay, he got back to me as far as what diff the truck has. He says its a 3.92 limited spin diff. I originally thought that it had the 3.55 so I'm glad to hear it has the 3.92, i would have likely changed it to that eventually anyways. I was under the impression that all half tons were shipped with the 3.55's, was 3.92 gears an option for the 07' 1500's?

 

Yep, if it's a whine coming from the different it means the clips have done their dirty deed to the gears. Although this whine could just as easily be a simple wheel bearing or pinion seal too.

I have found there is no rhyme or reason to factory gearing. Seen 2WD 4.7 reg cabs no tow package ship with 3.92s and QC 4x4 Hemi trucks with factory tow and heavy a$$ 20" wheels ship with paltry 3.55 gears...