Originally Posted by labradort

I've been in the garage and my mechanic believes it could be
the idler pulley under the alternator (not the one on
the alternator).

By the way, my previous mechanic had warnings about
cutting away the heat shield. On many vehicles (not sure
what it protects in the Frontier) this shields the alternator
from a premature death from heat. He told me about a guy
who replaced his and decided to forget replacing the
heat shield. A month later he needed another alternator.