Sidechain compression popping noise in Mainstage

So I'm trying to pull off the sidechain compressed synth bass tracks on my bands newer recordings we are finishing up and I figured the easiest way to do it was to trigger the sidechain with a seperate instrument in mainstage. I have my m-audio keyboard split so the lowest two keys trigger the sidechain and the rest of the keys are the synth bass sound getting compressed. My issue is that whenever I hit the keys to trigger the compression I get a popping sound and I can't figure out where it is coming from. The channel strip for the trigger is set to have no output and so is the bus it's on so I don't really know where to look. Ive tried hitting the triggering keys before I hit what's going to be the audible note to make sure it isn't the attack that is giving me the popping and I still hear it. Regardless of whether there is a note being compressed or not, whenever I hit one of the two triggering keys I get a pop. Does anyone have any idea where I should start in the troubleshooting here?

Just a hunch: have you tried assigning (also) notes to these two keys with velocity set to zero?