I have a Carrier 2 1/2 ton heat pump air conditioner. When the heat is on and it goes into the defrost cycle there is a very load humming and/or vibrating sound that last up to 15 seconds. When it comes out of defrost I hear the same sound for about 4 or 5 seconds. Is this normal? I have never heard any of my neighbors AC make this noise. It is so load that it sometimes wakes me up at night.

Thanks for any help.

Yes it is normal. Probably the old reversing valve making the noise.