A humming noise may occur when the Roland P-6 and other AIRA Compact units are connected to each other through the MIX IN/OUT jacks, and all the units are also connected via USB to the same computer.
This noise is caused by a ground loop, which can happen whenever there is more than one circuit path to ground. It is not an issue specific to AIRA Compact units.
To check whether the MIX IN/OUT connection is the cause, disconnect the MIX IN/OUT cables between the units and see if the noise stops.