Help on Aleph 5 buzz.

My new Alephs are sounding better every day. However I have noticed that although each monoblock is very quiet by itself or connected by itself to source when I connect both mono's to source there is a perceptible buzz that is higher pitched than the very slight hum you might normally hear if you are right next to the mid /woofer.
Nelson has said that a hum is a ground loop problem and a buzz is parasitic oscillation.
How do I go about solving this problem ?
I had no buzz or hum with mine! But seems it's a ground loop...

I have no earth linked to the preamp's ground.However,each mono block is physically with its own ground,each ground goes to earth through power thermistor CL60.

The only place the 2 grounds from the amps meet is the output XLR of the preamp :wiz: (there is nothing magic)

Thanks for the replies.
It seems that it is my fault for not putting in a thermistor in the connection to ground. In fact I have not put one on the power line either because my amps seemed to switch on perfectly well without it. I guess my inexperience is to blame. I did try lifting the ground on one of the monos and the buzz disappeared so I guess with the thermistors the problem will be solved.