High pitch squeal assending hills
 

I've got a 2001 f-350 V-10 that makes a squealing noise going up a steep hill until it shifts into a higher gear. I must also say that it only happens when pulling my trailer of 7,000 lbs. It sounds like a pulley or belt, but I,m a new owner and can't be sure. Any input would be helpfull.

I would have the transmission checked just to be on the safe side.