Hello all,
I have a 2004 Chev 2500 HD with a Duramax and Allison Trans. The vehicle has just over 5000 miles. I bought it to tow a 5th wheel trailer. When I'm towing and in TH mode I get a loud grating type noise under light accel and at times decel and cruise. It sounds like gear noise. I thought at first there was a harmonic resonsnce being sert up at that rpm and perhaps there were PS or Trans lines that were rubbing each other but I think I have isolated it to the trans. I can't duplicate the noise without towing. Anyone have any ideas on this one?

Garry G
Concord, NH

Does it happen at any particular speed?

I haven't heard it at less than 50 mph but it seems to me to be more rpm sensitive than speed.

That's a strange one. From my experience if you're grinding gears you'll hear it all the time whether you're towing or not. Is the noise constant at a certain RPM or is there just a quick buzz and then it goes away?

It is consistent between 1800 & 2200 RPM when under light throttle, cruise and at times decel.

Could be one of the tranny heat shields rattling but that doesn't explain why you only hear it while towing.

One thing you can try is park your truck and run it through that RPM range with the transmission in both neutral and drive. While you’re doing that have someone stand outside to see if they can hear the noise and isolate where it's coming from. Just make sure they're not standing in front of or under the vehicle when you do the drive part.

Liked I said it will only happen while towing.

Will the noise change with steering wheel movement?
mike

No change with steering movement. It is definitly rpm/load sensitive. I tried every way to duplicate the noise without towing my trailer but it just won't do it. There is no way that I have found to duplicate in the shop.

Is the noise steady or intermittent? My 05 LLY makes an odd noise similar to yours, but does it in any condition, louder when towing. I thought it was engine noise, but it could be the trans. Its going to the dealer next week for this, so I will let you know what they say.

Don't know if it matters, or is even the same kind of noise, but,

I had one of those transfer cases that got a pinhole leak from a missing keeper letting a pump move around. The shop that fixed it forgot a snap ring in the transfer case.

When driving around, it would intermittently make gear grinding sounds. It sounded like it came from the transmission, but it was the transfer case.

Bert

It could be the transfer case i guess. at this time there is no way to isolate it. I may have to wait till something breaks!