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.
Very strange..I thought I wrote a follow-up last week to your responses, but it isn't here. I'll try again. The transformer noise problem was reported to me by a couple of electricians who had replaced the noisy transformer, only to note that the new one acted exactly the same as the old one. That's when they noted that the ground electrode connection was missing. Once it was properly grounded, they claim that the transformer ran quieter. Do you suppose this was just wishful thinking? All I can think of was that there might have been some preferencial capacitive coupling for one phase, and it resulted in some magnetic imbalance...creating excessive noise...but I seriously doubt this is the case. I'm open for some enlightenment here....I would hazard a guess that the electricians just didn't let the new transformer warm up enough, or that the loading was different (as you suggested Bjarney) and they didn't notice that fact.. I haven't heard of the "cast coil" type...I'll keep a look out for them now, Nick.
Here is a link to info on Square D’s Power Cast cast coil transformers. I forgot when I posted that they are only available as unit substation transformers with medium voltage primaries. I installed a 1000KVA and a 2500KVA last Christmas and when you energize them, after the initial inrush, they are no louder than a small 15KVA step down. I have stood next to dry type subs that you had to yell at the person next to you for them to hear you. These cast coil were amazingly quiet. Like I said though, they are not cheap. They are top of the line and price list.
Gj, the transformer "repairman" told me that most transformers are extra noisy when the vibration dampers are too thin. Even if the bolts are backed off, the transformer still presses down through the thin pad and transfers some of the vibration to the enclosure. I'm still waiting to see what the final "solution" will be for the transformer we took in for him to investigate. Thanks for the link and clarification Nick! I suppose that the quieter the transformer is, the less scary it is to put on line. The usual dry type 150 KVA transformers can sound a lot like a large arcing fault during start-up! Gives me the creaps being around them for that first energization.
Found out that the transformer was defective from the factory. I heard that there was a recall for the transformers -- which weren't glopped together enough, so the core laminations were too loud. Seems that you have to incur the added expense of proving that the sound levels are too loud before the manufacturer will admit that they sold transformers that weren't up to specs!