Core Power Technologies

No properly designed power conditioner should hum. That one hums in response to a curling iron being used merely indicates faulty design or a manufacturing defect.

@ celander and anybody else for that matter. Has anyone opened up one of these units? I did!! In that little black box is a balanced transformer and a breaker. That’s it! No electronics or fancy crap. So if it’s a design issue, maybe that’s why? Thoughts??

Many transformers will hum under certain conditions.  Really good quality ones, produced under high levels of QC, usually don't.  A lot is in the QC.  If the winding is a little "off", an individual example of a brand with a generally good reputation may still hum.  I think that's why they thought they could solve the problem by sending you a replacement--if only they had.

Or a transformer might hum if it's being starved for current. you could measure your incoming voltage and if it's high that I could be part of the problem. 

@twoleftears I recall Mark selling off some B stock 1800’s having newly installed transformers. So there could be some truth to the transformer QC.