Usb Midi noise connecting to 4ms MetaModule

Hi all,

I use an MC6 Pro (Usb device port) to send MIDI to 4MS MetaModule (Usb host port). Everything works great, except for a high-pitched ground loop noise that appears as soon as I connect the two devices.

I wonder if this is happening because the MetaModule is powering the MC6 Pro. I noticed that the noise increases significantly if I disconnect the external 9V power supply from the MC6 Pro.

Any idea to solve this?
Alternatively, can anyone recommend a USB noise isolator/filter to insert between the two devices?

Thanx in advance,
Valerio