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I had my air handler serviced last year towards the end of the year. I used the air conditioner about twice after he had worked on it. It howled during this time but I didn't think anything of it since it didn't do it right after I turned the furnace on. This summer though I noticed it howls again so I did a little deducing. I found that it doesn't howl when just the fan is on. It doesn't howl when the furnace is on. It only howls when the A/C is running. I change the filter very regularly so I don't believe it is that. It drains properly since I clearly see it eject the water outside using a condensation pump.. The howling seems to be coming from a lower section of the air handler. Right above the fan where the furnace is. I'm fairly certain the sound is below the A coil. What I don't understand is if its coming from the furnace part of it why does it not make any such howling when the furnace runs. I'm including a wma file of the sound if that will give you a better hint. Any help would very much appreciated.