hello does it seem like your car is slowing down faster when the engine in humming then when it is not, and are you going fast than 50mph when the engine starts humming after you let off the accelerator pedal. if you say yes to both than it's fine. I dont know what the name for it is but i call it engine braking. just like the big hevy duty trucks use while going down a hill. nothing to werry about. but is you say no to bouth answers the check your car in a shop and ask them whats going on. P.S. to prove my point put the car in over drive thats the D after N go up to 65mph and put the gear to Drive thats the secend D. You can change from one to the other at eny speed. after putting your car into the secend D your car shud start the engine braking "the slight humming nois" have fun and drive safe.