It is 60Hz noise. Not all that uncommon, but more common when one of the connections is not solid. Can also occur when Flexwatt is covered by a conductor like aluminum tape. Magnetic field flows around it, manifests itself as noise. Grounding the tape can help alot. 3-wire plug - ground wire to the aluminum tape if that is what you used.

The hum is not dangerous, but darn annoying.
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Mark