G-Major Channel switch problem! Need help!

[mastermikko]

I just got my G-Major and Behringer FCB1010 today and I run them through my Mark IV combo. I use a normal guitar cable as a relay wire to switch between RHY1 and LEAD because those channels are all I need. BUT there's a little problem. When I switch back from LEAD channel to RHY1 the amp makes this big noise. A little bit like the volume would suddenly be much higher for a second.

Is there any way I can get rid of this problem? I read somewhere that the noise gate could do the job but how?

Thanks in advance!