DJM-A9 wobbly sound when using FX

I bought the DJM-A9 about a year ago, and I think the problem occured from the beginning.

When I try to use the FX I hear a wobbly sound in some cases. Here is a description of some of the cases:

1. When one of the frequencies is off (for example LOW) and beat FX is off and the FX level/depth is at MIN , I hear a wobbly sound when I turn the FX level/depth knob.

2. When one of the frequencies is off (for example LOW) and beat FX is off and the FX level/depth is not at MIN , I hear a wobbly sound when I change the channel for the FX.

The wobbly sound sounds like some kind of very fast cut/drop in the sound output. After that it occured it's also like there's a very small delay on the sound, compared to before I did the action.

s this an issue with all of these devices or is it a problem with mine specifically. What can I do to fix it? Because this makes it actually impossible to use the FX properly without hearing side-effects.

Thank you very much for your answer!