PLEASE HELP this car is a Mercury Mystique year 2000, 148k miles on it. was...(More)

PLEASE HELP this car is a Mercury Mystique year 2000, 148k miles on it. was my moms but my dad got a new car and my mom got my dads, and i got my moms. so my mom basically put this car through alot. but this is the only engine issue i have with it, everything else is body work. but when i rev the car to 3000rpms, it makes a loud humming noise that lasts for bout 15 seconds, then stops and the car's idle drops very much. sometimes the car will start humming when i put it in reverse or when its sitting at a red light. what does this mean? what kinds of things should i have the dealership look into?(Less)