Hi Jonyeo909. Do you say you can hear the filter self resonating at the normal level but the VCO's are quiet
If that's is the case then the Hi/Lo/off switch wouldn't be the candidate would it Florain
If the level of everything is low including the filter and I'm reading it wrong then Florain's suggestion is he most likely cause ;) get some switch cleaner in there and flick the switch back and fourth several times.
The CHD midi should interferer with any of the analogue voice circuits as it only interfaces with the key assigning board which is why we have the stepped pitch wheel implementation so I doubt the kit has anything to do with it but installing it may have exasperated and existing problem to a now noticeable level.