vacuum cleaner under the engine?
 

I've started to get this noise under the hood, that sounds like a vacuum cleaner when ever i accelerate, I had the mechanic check it out, he says it's the water pump, and that it needed to be replaced. I don't believe he was correct, My engine isn't overheating, and there are no leaks. Someone else said that they think it's my alternator. It only makes the noise when i accelerate.

98 disco
136000

Remove your belt and then spin things buy hand, do they make any noise?
With the belt off start the truck and then rev the engine over 1500 rpm a couple of times, does it still make the noise?
I must add, DO NOT let the truck over heat while you are doing this.
Only run it for a few minutes and with the heater on.