I've gotten a weird buzz/hum noise in my pickup. Only does it when the truck is in gear but gets much louder when the lights are on, slightly louder if the AC is on. If the truck is in neutral or park the noise goes away, if I shut the lights and AC off the noise goes away? Also noticed recently my day time running lights no longer work? Whether that's related at all (personally I'd rather not have then anyway). Where would you guys start looking?

 

Alternator.

 

I had thought that too, would it only be making noise in gear because there's a load on the pulley then or?