I'm concerned that it ran rough for the moment. But it may have not been enough to damage the engine. In some cases a severe heatup will have warped or damaged an interior engine component and will reveal it's failure under the most stress, and going up a steep hill, it the best test.

I've heard u-joints make this noise, and it follows most your description of "under acelleration," and "up the hill." But, the fact that it happened the day after the heat up, it's just too coincidental, especially for the milage the vehicle has. Also, on u-joints, to help test, we can add grease to them and allieviate the noise enough to tell if the joint needs replacement.

Form what you describe about the noise sounding like a belt, and the fact that the mechanic thought it may have even been one, leads me to believe the problem may be comming from the front accessory/belt area. The heatup could have affected one of the front components such as the water pump (most likely), alternator, power steering pump, a/c compressor, the engine fan (more liekly). Let me do some research to morrow morning and i'll leave you a more precise idea of where to go from here. Thanks,

-Rip