vibrating noise under the hood on passanger side

I have a 2003 CRV with about 40K miles on it. Recently it started to make a loud vibrating noise only when the air is on. When I look under the hood
there is no indication that a belt is worn. Any ideas?
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fishsticks wrote:

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Since the AC rad fan only runs when the AC is running, check the fan blades (carefully) to see if it's a balance problem (dirty or missing blade), or the upper rubber mounts on the rad may be loose. If you ever use a pressure washer on the rad, don't get too close, you'll bend the cooling fins.
YOu could unplug the fan temporarily to be sure if it's the AC compressor making the (seizing) noise. :-(
'Curly'
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