Alternator & belt change - Now hums at low speed

[ncollom]

I own a 2006 v6 2wd Sport 4th gen.
After swapping out 6 month old battery, battery light was still on so I swapped Alternator & Serpentine belt from NAPA. Battery light is out & everything seems to be working swimmingly 4 days later.
However, now I hear a hum at low rpms.
Any idea of what this could be?
My 4runner has 145k on odometer.

 

[MikeInNH]

When replacing the serpentine belt - you'll notice little groves in the belt. These groves MUST be aligned with the groves on the pulley's. If NOT you could get a humm..and worse case premature wear/breakage of the belt.