Zep theremin making strange noises

Hello, I am a guy new to theremin.

I bought a zep theremin online and I plug it in and connect it with my speaker with a 3.5mm to 1/4 inch jack adaptor. After I turn it on, I find a problem, even though I am not playing it, It is still making some strange noises. Is it normal to have this sound or it is a problem can be solved? Sorry for my bad english.

The Zep theremin is one of Jake Burns' products. Have you asked him about it? - he is a helpful, friendly guy.