2004.5 48re makes noise shifting 3-4 when towing
 

Hey all, I have a 2004.5 with a 48re and 141,000KM (88,000 miles)on it. I bought it new and it is driven mostly by my wife (lucky girl)..truck is unmolested. I had an 11' Lance camper on it and towed my 18' boat behind it with no problems. Recently bought a 28' Jayco at about 9000Lbs. when towing up hills (coquihalla in BC) truck pulls good, but when it shifts from 3 to 4th under load, a metallic sound can be heard that goes away right after it shifts to 4th. Almost sounds like torque convertor locks and sound goes away. Other than that shifts good. Transmission has not been serviced...90% of the mileage is grocery getting and school runs...
What do you think? Thanks