Theremin strange issues

I have a couple questions about strange noises coming from my Etherwave Standard with ESPE01. What can cause an Etherwave to have an unstabling warbling glitchy sound when the pitch is sustained? Could this be caused by an object too close to the volume loop? Also, what can cause a high frequency resonance / distortion sound almost like a metallic rattling? Both of these only happen sometimes. Thanks.