I use my 1996 3500 during warm weather times of year for pulling my 15500 lb 5th wheel. I purchased it new and now has 84,000 miles on it. I noticed last year that when boost gets to about 12 lbs or higher in 3rd, 4th, and 5th it makes a noise like a bunch of marbles rolling around in a container. I have changed the lube two times and used the proper Castrol Syntorq. I plan on changing it again in spring before I start driving it again to see what old lube looks like and if new lubes makes any improvement. Anyone have any thoughts on what the problem could be? It is RWD with a 3.54 axle ratio.