Noisy AC

I live in a two-story house and it's got a single AC unit in a half-closet upstairs. It can be fairly noisy in the dead of the night and I was wondering if anyone's successfully managed to minimize the noise caused by the fan when it comes on. At least I think that most of the noise comes from it.
The lower half of the closet, below the AC/furnace machinery, is made up by a box where the duct from the inlet vent downstairs converge in addition to a vent in front of it. The inside of the box is made up by naked wood and drywall, a pretty good resonance cavity for the fan noise. Or so I think, for the upper half of the closet doesn't seem to make much noise, even when the furnace turns on. Or at least the fan noise is far louder than the rest.
So I was thinking if putting some absorbent material against the inside of the box would help deadening the noise significantly. But any advice is appreciated.
TIA
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Augustine writes:

Your problem may be the fan (squirrel cage) is out of balance from corrosion. This happens over time due to corrosion from the condensate in the evaporator. It then vibrates or wobbles a lot and gets very noisy. Can be replaced, but I rebalanced mine by putting small binder clips (office supplies) here and there by trial and error.
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Oh, no, the noise is not wobbly or anything, just a loud blowing sound...
But thanks for the reply.
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Cover the walls with something sound absorbant for something quick ceiling tiles will help, you can contact supliers of sound absorbant material for more effective control.
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Impossible to say since we don't know the cause of the problem. You seem to want to treat the symptom. The real problem might be something like bearings that need to be lubed.
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Ceiling tiles inside the duct box is a good idea.
Thanks a bunch.
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On 13 Jun 2006 22:45:57 -0700, with neither quill nor qualm,

That's what I did in my last house with shag carpet samples; 3 walls and the floor. It cut the noise down by half (3dB). I also found that some media filters are noisier than others, so try different filters.
My new Carrier Infinity is TONS quieter and has A/C. I love it! 96% efficiency vs. the old 80%. I replaced the old 240v electric baseboard abortion when I moved to Oregon. The $6k system was well worth it. I'm in love with forced air because it's so comfortable. The baseboard system left your feet frozen and your forehead fried. WORTHLESS!
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