My Mackie HD1531 has a high pitched whistling sound, all other speakers work fine with the same power connection and desk connection. It whistles regardless of whether it has a sound input or not
Music - Mackie - HD1531 3-Way 15" HD Powered PA Cabinet
Answers & Comments
Sounds like a problem with the horn amp in that cabinet. This is not a user fixable problem and needs to go to the shop. Here is a link to the Mackie support page where you can find a link to the repair shop list.
Mackie Tech Support