With the engine running, get a long screwdriver or rod of some sort & VERY CAREFULLY place the tip on the alternator casing, idler and other items driven by the belt - put the other end to your ear - you will hear a much better indication of what is causing the noise. Better still get a mechanics stethoscope but a long screwdriver works fine & is what I use.
I stress - do this VERY CAREFULLY - do not let whatever you are using slip onto the belt or get entangled for obvious reasons - you will get a good read on the noise from touching and listening - you do NOT need to place the end right near the belt - simply on the suspect component will work just fine.
This is a time tested & accurate way to identify the source of noises in a belt train like this appears to be.
PS - I accept no liability for anyone who does not take sufficient care carrying out this procedure & if you are uncomfortable with this basic diagnostic I'd suggest you take it to a professional.