Minimum theremin 101

I recently built a Minimum Theremin 101. 
I am getting a little frequency drift, and also intermittent spurious oscillations, low level, but annoying. Sounds like radio interference, very low level whines whistles and growls. The main thing is the drift. The zero point changes slightly as it warms up. Maybe a circuit change could reduce or eliminate this?

"I recently built a Minimum Theremin 101. I am getting a little frequency drift, and also intermittent spurious oscillations, low level, but annoying. Sounds like radio interference, very low level whines whistles and growls. The main thing is the drift. The zero point changes slightly as it warms up. Maybe a circuit change could reduce or eliminate this?"  - Cattattac

If it's this one: https://www.harrisoninstruments.com/101/101_schematic.html

That's an RC (resistor & capacitor), rather than LC (inductor & capacitor), based Theremin.  The resonance can't be as selective, so I wouldn't be surprised if it's receiving RF.  And the resonance can't be as stable, hence the drift.  I haven't researched RC Theremins at all, but I don't believe they can be first class instruments.  The LC oscillator is a sweet spot for Theremins: stable enough, yet pullable enough.

The power supply to the oscillators is regulated, that's the main thing you can do to stabilize any oscillator.