Here I have developed a free software (GPLv2) percussion synthesizer. The latest version is 1.6, runs as LV2 plugin and standalone on GNU/Linux for now. It has 3 layers, with 2 oscillators+noise per layer. It has envelopes for amplitude and frequency, filters cutoff. The realtime visualization of percussion graph. Can export to FLAC, WAV and ogg, save /open preset in JSON. I hope you'll find it useful.
Here is the source code: https://github.com/quamplex/geonkick (https://github.com/quamplex/geonkick)