external hd noise (?)

Last week I installed an external HD 200 gig for extra storage for my images. It is a Maxtor hooked up by USB in a Bytecc Ext. Enclosure/built in power supply, two fans to cool it, etc. This Unit is quit and seems to be working fine except when I turn on my speakers there is a back ground buzz/hum. I know it is from the HD unit or power supply because if I switch it off when leaving the speaker on the humming stops.

Of course this is not the answer I am looking for -

Can anyone tell me how to handle this problem or direct to ending the speaker noise?

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KimR