When the air conditioner is running at night I hear a knocking noise. It is a hollow sound like slapping your hand lightly over an empty glass bottle. It is more frequent some nights and totally absent others.

Sorry but I cannot be of much help on this one. I looked all over the house for a glass bottle so I could duplicate the sound but and all I have is plastic.

Does it do it during the daytime?

I once new a guy with the same problem, turned out to be rats scratching their fleas