Today while pulling our 5th wheel we came to a grade where I used my tow haul mode to control our speed. When the transmission shifted we got a loud flapping sound we never heard before. It sounded like part of a recap on a tire came off but as soon as I shut off the tow haul the sound went away. any ideas?

 

Were you going uphill or downhill?

What were your boost levels?

Sounds off hand like it was bleeding off excessive boost.

 

Our truck is an 08 450 with the 488 gears and we never use the tow haul mode unless we have a down grade and want to control our speed. Trailer weight is 13,000 + lbs and we keep our speed between 55 and 60 mph. I don't remember looking at the boost gage but since my foot was off the throttle it should have been 0, right? The hwy we were on is 65 between Springfield and Branson Mo.

 

Yes Boost near or at 0. I think it may have been bleeding off excessive pressure.