In our facility, we had a couple 2000 kVA 4160 to 480Y/277V 3 Ph transformers that were purchased as replacements for the original GE units that had failed previously. This all took place years before I arrived. I put in place a PM program where we checked Megger readings, and took Turns Ratio Tests. The loud units checked out fine, they just "sang" louder than most. The NETA guy said that they were just "cheaply made". They continued to perform for years. One even worked fine after being single phased by the utility for an hour during an ice storm. Subsequent testing showed that other than 1 shorted turn in 1 coil of 1 secondary, it was OK. A couple of years ago, we got all new primary distribution, and new unit subs. Now everything is CH/Westinghouse and all is good. Your situation would appear to be different in that you supplied this unit [??], and it may be in your best interests as a contractor to change this unit out.
As an aisde, whenever possible, we try to purchase "Cast Coil" transformers as they are VERY quiet, and fairly impervious to many things. we have them from .25 to 2500 kVA. Only problem, $$$ and their weight.
