Humming noise at slow speeds

I have a 1998 Acura 3.5 RL
The engine is absolutely quite - when on neutral. i even tried putting car on neutral & with brakes pressed increased the RPM till 3000 - and
there was no vibration.
When the car accelerates from 0 to 30/40 .. the gas pedal feels little stiff .. and hear a wierd humming sound.
When i stop at a traffic light - the car vibrates a bit.
do you think the problem is with the transmission system? should I consider like a transmission flush, etc?
any other potential suspects?
would appreciate your response
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