Hello

There are two noises: a 50Hz "buzz" from the power line and real "noise".

The buzz sounds more like it is induced by neon light or something
similar. But it might be the transformer itself. I can't judge that from
here.

The noise seems to me the normal noise of the synths output (it _is_
noisy to a certain level), but the internal signal seems to be low. That
is normal for 50% Level. Use 100% Level for VCO1 and make the recording
again.

Basically: the signal level in the MonoPoly is that way laid out, that
you can work always with all VCOs at 100% level. There is plenty of
headroom. If you do not use this headroom, you will hear the noise.

Florian