I have been called to troubleshoot a couple transformers lately due to humming. I don’t have enough experience to pinpoint the exact cause for them. The first was 112.5 kVA delta 480 to wye 208 and it was carrying A-22a B-19a C-9a and voltage was with 2 volts to ground. The only thing I saw that would mess it up was the imbalance of load. The other was a 225 kVA transformer and we were putting a fluke power logger on it to see if we could find the reason for humming. Same setup as before delta to wye but it was feeding a 400 a panel which is essentially a 200 with a feed through 200 and a separate tap off the transformer to a 200 a panel. It was loaded A-10a B-6a C-4a all within 1v to ground but the power factor was A-99 B-85ish C-71 depending on the active load. Would the pf cause it to start chattering badly? If not what would cause it to with such a low load?