99 2500 GMC, che che che noise

My 99 GMC makes a che che che noise when I am pulling my RV trailer up a grade, seems to happen only on very warm days, will be ok when first starting up the grade ,but in a few min. it starts making this sound.
doesn't change with engine rpm, and it does it in all drive ranges, it varies with speed of travel, I have an automatic, sounds to me like it is the transmissin.
after I get to the top of the grade the noise stops but starts again when I hit another grade. I have taken it too a GMC tech, they found no problem and had no clue what it was.
camping season will soon br here and I would sure like this problem fixed before it cause's a breakdown. any Idea's