I drive the 2001 Tribute LX V6, 3.0. Last night I was on the highway headin' home, and I went to accelerate to go around a car, and the car didn't accelerate, it just started humming. I pulled off the highway and now my car hums loudly when I'm in gear, even when I'm not moving. When I am moving, it hums until I top out according to however much pressure I have on the gas pedal. Also, when I put it in park, it revs normally and the humming goes away, however it sounds like theres a slight knocking somewhere near the starter area, it lingers when I shut off the car, so its something thats still turning.
We tried replacing the starter, my uncle thought it may not have been disengaging all the way, but that didn't fix it.
Any ideas?

EDIT: More of a whining than a hum, sorry.

 

Manual or automatic?