Interference with DDJ-SB

Hello.
I'm using the DDJ-SB since 2 year. One year ago the interference appeared for the first time but after 2-3 weeks it was gone. Now i've got the same problem again. It's like a whistle not a growl. The sound changes for a moment, when I'm pushing some buttons on the DDJ... I'm always using the same setup (Mac, Serato Intro). I've tested it with ohter cables, computers and ground loop isolators. I've heard that the charger could be a reason, so i've tested it without too. After a measurement it's beginning at 1000 Hz. It's getting better, when I put the DDJ-master to the maximum and the DJM-master a little bit down (tested with a few DJMs). What can be the reason? What can I do? Please help me. Thx