Microphonic Jack Socket
It's been far too long since I've uploaded a video, I know.
This is a bit of an odd one. I was converting a pedal matrix from unbalanced to balanced audio, and had to order 20 1/4" jack connectors along with another set of relays. When I reassembled the box, it was very microphonic on certain jacks. I initially thought it was the relays so I ordered some more. Replacing them didn't change the problem. I looked further at the connectors themselves and found this.
Note how the socket itself isn't connected to anything. If you rub it on a rough surface it reproduces the sound pretty well. I'm thinking it's a problem with dissimilar metals, but I'd like to know anyone else's thoughts!