This is a module for producing metallic percussive sounds or discordant drones.
It’s based on ideas presented in a Hackaday article by Elliot Williams. There really are two separate circuits here. The first is essentially Williams’s “cowbell”, though with a 4066-based pseudo-VCA instead of the two-diode one. It uses 1/3 of a 40106 and 1/4 of a 4070. That doesn’t leave enough on those two chips to implement the “hi hat” circuit so instead I made something sort of in between the “cowbell” and the “hi hat” using the remaining 40106 and 4070 parts. Both circuits have a switch to bypass the envelope generator and “VCA”, giving drone outputs.