Engine compartment noise

I've got a 2010 V6 with 51,000 miles on it. Lately I've been hearing a noise that seems to follow RPMs. I'm thinking a bad bearing in something like the alternator, air conditioning compressor, tension idler or belt. But, it doesn't have a metalic sound. I remember my old Tundra had a noise and I changed the belt which fixed that problem. But this noise seems different. It's hard to explain but I thought I'd post to see if this might be something that others have already experienced.
How many miles (tipically) before needing to change the belt? If it's due for that I could change it and at that time be able to freely spin all things that spin to see if there is a bad bearing on one of those components. I seem to remember taking apart my idler/tensioner on my Tundra and regreasing that assembly. Any wisdom would be appreciated.

 

good rule of thumb for serpentine belts is 60 thousand miles. its cheap insurance considering it turns everything under the hood and if it breaks youre done. take it off and check the pulleys at least...i would ...nothing to lose