Audible Midi Signals

I can often hear clicks from the midi note on/off signals when i'm playing back a sequence or just playing from my controller. Can anyone tell me what might be causing this? I thought grounding issue, but someone told me that since midi cabling is optically isolated that shouldn't be the problem. I know for sure the problem is caused by the samplerbox in my setup. When I mute the sampler box on my mixer the clicks go away. At one point I had an issue where midi clock was audible, which I fixed by connecting the unused cv sync on my volca sample to my vocal fm (doesn't make sense), clock source is another raspberry pi running seq24. Any hints? It's a real bummer because it ruins any recordings i try to make. no noise removal techniques i typically use can get remove the noise without ruining the audio quality of the recording. Please help! I love this little thing and really want to integrate it into my setup. I'm using Amosic usb midi interface and a behringer umc204hd

I have same problem when I use knobs transmitting midi, even if nothing is mapped to Samplerbox... just the midi information creates clicks... I believe it might be because I am using a cheap MIDI interface (the casic from ebay with musical notes) which hasn't got the opto coupler or something?