Velodyne ULD-15 has a high pitch noise

I have a ULD-15 sub and when you stand it on its feet there is a high pitch sound coming from it. When you stand it on its side the sound goes away. I disconnected the RCA cable and the sound is still there. Can someone please help me with this problem?
Call David Santos at Velodyne, he's the ULD expert. His direct number is (408) 465-2819. His E-Mail address is;
Good Luck,