An ungrounded neutral [X0] terminal shouldn't on its own cause noisy operation.
High line voltage will cause transformer iron to run closer to saturation [high magnetic flux] and may force increased mechanical vibration in the laminations, making them noisier. [A 120-volt circuit should not run over 127 volts for sustained periods. Scaling this ratio to respectively higher line voltages, for a nominal 480-volt system the upper limit should be 528 volts.] One way to keep a transformer farther from saturation is by adjusting primary taps to compensate for increased line voltage. In low-load situations, line voltage typically runs higher, and there may be equipment shut down that would normally dilute transformer noise during business hours or production schedules.
Harmonic currents (mostly load dependent) and harmonic voltages (from the source and interaction between other non-linear loads and the source) can be a significant source of transformer noise, in the same way that incandescent-lamp filaments may “sing” when controlled by SCR-type light dimmers.
Sometimes noise levels can be aggravated by placement with respect to adjacent walls and other architectural features, acting like a megaphone. Humans can be more sensitive to noise depending upon use of an area. A noisy transformer that may be ignored on a production floor could be unbearably disruptive in a library.
For equally sized oil-insulated versus dry-type transformers, oil typically is quieter.
