Loud squeal at 30 pounds of boost

[f6maniac]

I have been reading the forums for the last 6 months and appreciate your knowledge and recommendations. I recently bought a 2006 6.0 F350 FX4 dually with 26000 miles on it. Took my 14000lb fifth wheel out this weekend and while pulling it up a long grade I intentionally pushed the engine. I was running at a steady 30 pounds of boost and heard the fan cycling on and off as expected. As I neared the summit, the RPMs fluctuated about +/- 400, and I heard a load tone. I didn't notice any power loss. I let up on the peddle and the tone stopped. It didn't repeat after I crested the hill or on a long flat section after. I had one more fairly steep but short hill to climb, maybe half a mile long, as I approached the summit I again heard the tone until I let off the throttle. Any ideas? Engine, tranny, maybe torque converter? I hate to take it to the dealer unless I can recreate the problem for the service rep. I haven't gotten it to act up without the trailer in tow.

Added: I have the 4:30 rear end and was in tow/haul mode.