2001 Toyota Camry Vibration felt under the Gas pedal floor

Engine Mechanical problem
2001 Toyota Camry 4 cyl Two Wheel Drive Automatic 70K miles

There is a growing Humming sound and vibration coming out from under my wifes Camry when driving. It seems to be at its worse between 45- 55mph. I first thought it was a wheel bearing or front suspension but my mechanic said it was not. He could not pinpoint the issue. I now think it could be transmission or drive train in the front.
It started out 6 months agao with a very light humming sound that no else could hear but me but now it could be heard at higher speeds and I can also feel it under the floor.
Everything else seems to be working great with the car.
Bob