Engine Air Leak???

Hello,

I have recently noticed a new sound coming from the engine of my 1996 Blazer 4.3. It sounds like it could be some kind of small air leak (???) -- it's almost a whistling sound; not quite a squeal...

I haven't noticed any difference in performance but the noise does increase with RPMs. It's not really loud, it is most noticeable while sitting at a light, where there is not much other engine or road noise. It's just something that's new, and obviously not supposed to be there so I would like to get it fixed.

I have also heard that the belts on these blazers can make a lot of noise -- could this be the problem?

Any ideas?

Thanks

Z