I have an Alesis Multimix 8 USB mixer and a Behringer Ultratone K1800FX Keyboard Amp. When trying to connect the amplifier to the mixer, a deathly annoying high noise pitch is heard. It is not very loud, but it is heard sometimes over my guitar. It is especially annoying for recording purposes and I am looking to somehow get rid of the noise. I have also tested that a direct connection to the amp would not make this noise, but only when connected to the mixer it would make the noise. It could be possibly how i am connecting the amp and the mixer which is by an ordinary instrument cable. Directly from one of the inputs of the amp to the CTRL RM input in the Mixer. Can somebody please help?

Thank You in advance.