humming problem

[4Run97]

while inside my 97 t4r and usually when the air is on..sometimes A/C..a humming noise begins when i am stopped and as soon as a i step on the gas it begins to fade away. anyone have an idea where it's coming from

 

[a77boy]

I had a 97 that did that,, turns out I had a leaf caught in the fan blower. Made the noise all the time, just couldn't quite hear it at highway speeds