I have a '97 with 128K miles. Had the coolant system flushed and refilled. Now, when the engine is cold, it sounds as if I have an exhaust leak (I don't believe I do). As the truck warms up, the noise goes away. When I pull out of the driveway and accellerate, the noise is noticable for a few miles until the engine has really warmed up. After completely warmed up, the truck does not make the noise when in park and the engine is revved - only appears under load (when warm).

What could be the problem? The first thing that came to mind was water pump.

Thanks!