You are getting harmonics. This happens with proportional thermostats and some setups. The pulsing of the electricity at certain frequencies next to metal cause the hum. Occasionally metal rack systems experience this as well if they have a metal plate underneath the flexwatt. You can change the mode of the Herpstat to NON-Proportional and it will regulate within a half degree and no longer cause the hum otherwise you are looking at removing the foil. I would recommend not to have foil around flexwatt anyway. When flexwatt fails it often melts through the plastic layer. The foil can conduct and carry the current which could be a possible fire hazard. On a rack if it touches the metal the rack is grounded so it should flip the house breaker.
Dion Brewington
Owner, Spyder Robotics
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