I think that "potting" is pretty standard, since that process is the main factor in keeping a transformer quiet, but I may be wrong.
All things being equal, and the seller being honest, any noise the transformer could create would not be due to a deficiency in its' construction, if it has already been demonstrated to be quiet. I'm fairly certain that a noisy transformer cannot (easily) be quieted, although a quiet one could certainly become noisome in the right (wrong, electrically) environment. I have found that in my own system, the electrical supply to my residence made all the difference. I now live in a young condominium which supplies my unit with a very stable supply of 208 volt current. Everything is always very quiet, regardless of what appliances or coifing tools my girlfriend is using. At a previous residence, our entire cul-de-sac was supplied by one transformer, and that created all kinds of irritations when listening to music.
I suppose what I'm rambling on about is whether you can determine if you'll have a problem or not, before you make a purchase. In my mind (young, fallible), the following factors make transformer hum more likely:
1) Many homes drawing from one power transformer (on the pole) which will create fluctuations in voltage (before your breaker panel)
2) Current supply to your home in particular (low capacity) : e.g.- my condo has a 200 amp service, which I could not come close to needing, and holds perfectly steady at 117 volts no matter what ( I even put my lab scope on it for a few hours ). However, many older homes only have a 100 amp service. So, with upgraded appliances, lighting, etc..., it sets the table for large fluctuations in voltage, and thus, noisy transformers, among other things.