Here's a new one...70mph grumbling

sdeeter19555 · May 30, 2013

I've been driving the 2500 since my DD is out of inspection for the moment. I drive 100 miles a day round trip, mostly open interstate. I've been getting ready for a camping trip, truck has a partial load of firewood on, tires are at 70psi on all four corners, just had them rotated and balanced. I thought I noticed what to me sounds like a bearing going bad the other day, but it only makes itself present at 70mph or better, and under a coast.

I noted it again this AM in cruise, then started seeing if it was the radio, the AC, if it changed with speed, etc. Under 70mph, its quiet and smooth. It sounds/feels like its coming from the rear of the truck. Running low speed along a jersey barrier there are no odd noises coming from underneath (you get the echo off the barrier). The type of pavement makes no difference, so that sort of rules out tire noise. Mileage is consistant, low to mid teens. And I can swear I can almost feel it, which leads me to the differential (if it was a ujoint siezing, I would expect it all the time). I just changed fluid in the differential (75w140 Amsoil SVO) about two weeks ago, didn't note anything odd until now.

I a little concerned because I leave tomorrow afternoon for a 3 hour ride into the mountains of PA for a camping trip, towing right about 4,000 pounds. I'm going to pop the plug on the differential, see what condition the gear oil is in (anything obvious). Anything else?

Silverado4x4 · May 30, 2013

Why would you use 75W140?. Recommended is 75W90. 75W140 is a little heavy for that. I would drain it and put what is recommended in there.

pm26 · May 30, 2013

You should use 75W140 only in extremely hot states during summer months or when towing heavy loads. Heavy gear oil helps quiet noisy differentials somewhat, but that is not its purpose.

sdeeter19555 · May 31, 2013

The Corp 14-bolt isn't finicky as far as weight of oil, besides when its cold 75w140 has 75 weight characteristics just like the 75w90, and at operating temperatures it isn't any different than a 90 weight running slightly colder. I tow right around max GCVW once or twice a month for a couple hundred miles, this camper is light weight.

I figured out the issue...the Bridgestone Duravis tires, which I hate. And they only have 17k miles on them, I can't wait to be able to put a different tire on this thing than what came from factory.

I got home and checked all the fluids, figured if something was letting go somewhere; there should be metal flake in the oil...been there done that. Everything checked out fine (trans, transfercase, rear differential). I even ran my hands along the drive shaft to make sure there wasn't a missing weight or ding somewhere. There was nothing showing anywhere...and a bearing should be present itself at all speeds, to some extent.

I then checked the tire pressure...which I had set cold the other day at 70psi, it was almost 80psi hot. I dropped it down to what I had been running: 65 front/60 rear (hot) and there was no noise this AM (I still get it a little bit of the noise, but now it changes based on the road surface). I have never had a set of tires turn noise on/off at a certain speed before, or have a range at which they make noise.

I can only guess that between rotating the worn fronts to the rear that is already plagued by frame beaming, and then jacking the pressures up to a point I had never had them before; the tires were running on a different portion of the tread face (which caused a noise I had not heard before). That, and I wonder if there was some resonation inside the cab (similar to exhaust drone) going on at that speed.