Today I was towing a trailer uphill into the mountains. I wasnt getting on it and then all the sudden it sounded like an electric motor spinning out of control (or whining). It was extremely loud. The sound went up and down with acceleration but it also seemed like the trans was slipping. Trans temp was 185 and while it made that sound the temp went up to 200 then back down to 190 when it stopped. It definetly is coming from right under the floor board. I have never heard a sound like that except from a Jet. Any Help Please....

2008 Chevy Silverado 2500HD Duramax, Allison 10K Miles

probably the cooling fan

deffinetly the cooling fan x2...also reason you probally think your trans is slipping when you hear the fan is because the fan draws alot of power out of the motor so it feels like you lost power to the rear wheels, and technically you did.