Hi I have a 2004 base model mustang and it’s been making a humming sound at idle which can be only heard from inside the car. Gets louder on a cold start, at night, when I turn on the headlights or step on the brakes and come to a stop at a stop sign or light. Took it to two different mechanics and neither could figure it what’s going on. My battery is in the normal range but on the lower end of normal. One said maybe the alternator, other said PS pump and pulley. But the car doesn’t make any loud noise on turning. I’ve already replaced the belt. Can someone please help me figure out what maybe the cause. Thank you!