when connected to USB AND HIFI docking station makes buzzing noise

When I connect my new docking station to my hifi AND at the same time to USB (from my PC, Mac, NAS, Router and the Apple USB Power adapter! I tried all) there is a terrible buzzing noise.
If I connect the audio cable directly to my iPod or another MP3 player all is OK.
If I disconnect the USB, all is OK.
I replaced the audio cable.

I only have one iPod, one hifi and one docking station.

Does anyone have an idea how I can fix this problem?

thanks

gert