I think i have exactly the same problem as you bumble bee!
It only happens once it gets to its normal operating temp and prettty much only at idle, its not just one tone either it changes up and down.
I got my belt installed by a professional mechanic that has done heaps of belts before so i doubt these guys would tighten all our belts to tight, but im going back to get him to loosen it anyway so i will let you know how it all goes.
If the noise sounded like a supercharger or something it would be ok but it sounds like its dying or very sick.