I have an ML320 1998. A couple of weeks ago i started hearing a humming sound when I would press the gas pedal.
Once you release the gas pedal it stops even if the car is still coasting. When I put it in park and pressed the
gas pedal, it still makes the humming noise. Is this a transmission problem? Please help!

 

It sounds like bad bearing somewhere ( in alternator, serpentine belt tensioner or in the transmission). Also could be loose cat converter blockage or vibration resonance.
You forgot to state how many miles is on your ML320.
Bernard