UPDATE:

For shits and giggles I decided to spray a small amount of Valvoline Penetrating Spray Lubricant (love this stuff) on the belt closest to the firewall. Instantly the minor whistle vanished. So I'm thinking either my belt(s) need to be tightened/aligned or all that road salt dried out my belt(s).

Either way, I'll be coming up on my 60K mile service interval in 9K miles and they'll be covered then.