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Strange sound

I have a 2003 honda pilot that’s been making an intermittent grumbling like noise while driving between 25-45 mph on flat road and with very low torque being applied(driving to just maintain speed. At first the noise was barely audible and only my wife and I could hear it. It has slowly worsened so now two mechanics have heard it. One wants to let the tranny guys take a look, the other thinks it might be because the rear tires have substantially more wear and because the vehicle has 4wd capability (very limited ATM-4 system)the difference in the circumference of the two pairs of tires is causing the differential to turn under stress. Theoretically makes sense to me but would seem the size diff would have to be pretty large for it to affect anything. Any thoughts would be appreciated

The differential theory is a possibility, but as you know, since the AWD runs only intermittently, it is less likely than with a vehicle with full-time AWD. Of course, that does lead to the question of why there is such a drastic difference in tread wear on two of the tires. The likely reason is failure to rotate the tires on a regular (every 5k, or 7.5k) basis, and that is not a good thing on any AWD vehicle. A long-term difference in tire circumference could lead to this type of problem.

The transmission is also a possibility, and that leads to another question, namely:
How many times have you changed the trans fluid?

With a vehicle that is at least 7 years old, the trans fluid should have been changed at least twice so far, on the basis of elapsed time, but since you have accumulated 140k miles on the odometer, the trans fluid should have been changed at least 4 times so far. If you have not done this type of maintenance, trans failure is in your immediate future, even if that noise is not coming from the transmission. You could try a “Hail Mary Pass” trans fluid change after ignoring this service for many years, but this tactic rarely works.

Another possibility–and one that will cost a whole lot less money–is a bad wheel bearing or two.
Does the noise change or go away when you take a curve at 25-45 mph? If so, that is a pretty reliable indicator of wheel bearing problems.

Also–rear differential problems on these vehicles tend to result from failure to change the diff fluid every 60k miles.
If you have not done this, make sure to do so, using ONLY genuine Honda Dual-Pump Fluid.

Overall, this vehicle may be suffering from lax maintenance.
Is that a possibility?

wow that was fast The diff fluid has been changed and according to both shops is currently in good condition. The front tires are newer than the rears. I guess even though the vehicle is not a true 4x4 all tires should be changed together? The 4x ability is very limited–to help pull you from snow or mud, 15-20mph max, short duration. Maintenance has been above average but not with dealer

“I guess even though the vehicle is not a true 4x4 all tires should be changed together?”

Yes, and then rotated consistently.

What is the actual maintenance record of the transmission?

Please give my suggested test regarding the wheel bearings a try, and then report back to us on the result.

Fluid changed @ 60k and again @ 120k. Could it be a wheel bearing if the sound is not constant?

Yes, wheel bearing noise can be somewhat inconsistent in the early stages, especially during differing temperatures.

IF you havent already gotten this fixed… I can tell you that I think it has to do with your Torque Converter. I had a 2006 Acura MDX that I believe shares the same Transmission and motor as the pilot… started having this EXACT same thing happen (everything you detailed) at about 60K…and it was the torque converter…and there was a recall. Not sure if this recall encompassed the Pilot or not. But that is definately what I would think it is.

Wait a minute…yo have different diameter wheels on this car? Different circumference? Oh I see…its because of the tread difference…This is a possibility…as this car would NOT like a difference in the size of the tires AT ALL… Im not sure what the threshold would be however…it may not need to be that big of a difference at all.

If the vehicle sensed a set of wheels were turning faster than the others it would try to engage the transmission in a way to gain momentum from the other set of wheels…Yes this is a very real possibility here I think. It is able to throw torque at any set of wheels it wants quickly and I believe electronically…so this is important.