I have a pair of Monarchy M-150 Power conditioners that have a common problem they both have a massive amount of Hummmmm when plugged in so loud infact you can hear them from 10m away and to the point I just can't use them.
These are massive units 75lb each and configured for 240v to 240v as originally I had an issue with regards to over voltage to the house because of a transformer issue from the power company supplying 256v at the house and this is with over 500m of underground mains from the meter box.
I have been in contact with the supplier and then the manuifacturer (Eaton as it turns out) but with no result, currently I am running these into ( or was ) a pair of Hydra V-Ray conditioners with all connections to the equipment with BDC power cables.
The connected equipment is totally silent but the noise from the conditioners is the issue, i have checked the mounting bolts, fitted silicone matting under the Toroidal transformers checked all that i can think of and still no result. Both the supplier and manufacturer I don't think believed me as to just how loud this hum is, done the burn in thing Hot Cold doesn't seem to matter.
Tried different power points nothing, the units did in fact address the overvoltage issue as the power out is exactly 240v rock steady it's just the noise.