Macbook Pro high pitched noise


New Member
Have my 2012 Macbook Pro connected with two USB Audio Interfaces, each one on its Macbook USB port:

1- Roland RD-2000 (internal audio interface);
2- NI Komplete Audio 6.

The output is to a pair of Yamaha's HS5 active speakers.

When only one audio interface is connected to one Macbook USB port, all is well.
As soon as I connect the second audio interface to the second Mac's USB port I hear a high pitched noise on top of the speakers normal hiss. If I disconnect the cable, leaving only one USB port connected, the high pitch hiss goes away.

Any help on solving this ?
Thank you.