I just bought a Hamilton Technology 16" PC 2x28W ABS hood with one fan:

The fan itself is not so noisy, but because of hood echo or whatever, there is a loud low frequency hum coming out, very annoying.

I called HT sales support, they said:
The fans on our PC systems do have a humming sound that accompany the fan due to the small size of the ABS hood.

Is there anyway to reduce the noise? I placed my hand at the air-out side, sort of covering the air output, the sound reduces a lot! Can I put a filter pad at the air in-take? I'm afraid I can't put one at the air-out side as it will prevent the air from coming out - although I think it will definitely reduce the noise like I put my hand to cover it.

Any idea will be very much appreciated.