HIGH PITCHED SQUEEL???

Well here's the problem when i'm loaded up 10.5 camper, pulling 18ft trailer loaded approx. 6000lbs pulling a long grade and the pyro temps get around 725 and up but never exceeding 850-875 i get this very high pitched squeel.Back off the throttle it dies down put your foot back in it comes back up.Once you crest the hill and cruise for a mile or two and the pyro temps come back down 550-600 approx.noise goes away.It seems you've got to be into the hill for a couple of miles and the temps over 725 for a bit before the squell comes on,while doing this though there's no loss of power truck pulls real strong,what do you think??(TURBO ON THE WAY OUT??)