Transmission Shifting with a slight Thump

[LuvRving]

Hi,

Maybe I’m just noticing it now but my 2012 GMC 3500HD Diesel sometimes makes a thumping noise when it gears down when going up hill, I guess it’s from 6th gear down to 5th. It doesn’t do it all the time and it seems to be a bit smother if I’m using cruise control. It does it with or without the trailer in tow. I use tow-haul mode when I’m towing. It pulls well and shifts well otherwise. The trans temp is always acceptable and the trans fluid level is OK. There is 112000 KM (70000Mi) on the truck and the trans fluid and internal filter was changed at 70000 KM (44750 mi). I always have the external spin-on trans filter changed at every oil change. Is there anything here I should be concerned about? I don’t want to ask the dealership about it and be swindled into a major repair.

Thanks!
Joe

[Roger]

I'd take a look at the u-joints.

[EAW]

Also consider checking the condition of the rubber around the driveshaft's center carrier bearing. You could also notice a vibration when pulling away from a stop while towing.