An EMI filter does not filter DC. Likely the heater is phase controlling the AC line to control the amount of heat it produces - lots of line harmonics at 120, 180, 240, 300 hz and higher harmonics and your power supplies are buzzing from them. High value cap like 5 or 10 uf X2 rated UL approved line cap might help filter the source if placed near it, but no gaurentees
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