It sounds like you have a vaccum leak. I'd check the vaccum advance on the distributor first, then all other terminations.

The way you described the sound of the hiss (stops under acceleration I believe) is typical of a vaccum leak.

Since you recently had the water pump removed, I wouldn't be surprised if someone had to remove a vaccum line (to get to the pump) and forgot to put it back on, or they accidentally pulled a line off while R&Ring the pump and failed to notice it.

If not that: Check each pulley separatley. Remove the belt and spin the accessory pullies by hand if you can. As the last poster (I forgot who it was now) mentioned, you can typically 'feel' the worn bearings also. If the pulley has more than a little wobble in it, chances are under varying loads and high RPM's, the bearings will tend to 'sound' different.

I got a feeling its a loose vaccum line (they suck! ;).

Good Luck,