Could be mains noise. I get this on my Avantgarde Duo horns drivern by Quad IIs. Late at night is much better as I don't pick up mains noise from either electrical appliances in my home or even from other houses up my street.
Noise from tweeter
I am using Aerial 7B's drivin by a ML 333.5 amp from a Theta Casa Nova and a Sony DVP 7700, I am using Kimber Monacle XL to Aerial's, Kimber Hero interconnects from Sony and Kimber Hero balanced to ML. When I put my ear to the tweeters I can here noise, even when evrything but the amp is turned off. The amp is plugged into a dedicated 20 amp circuit. Note: I had the same problem with the Classe' CA 400 I was using before. Thanks in advance, Henry