My 1st tube amp: transformer buzz audible 15 feet?

It sounds like a defective transformer. Is this unit new? Who makes this unit? A mild buzz is not uncommon in a unit that's a few years old.

I have a pair of monoblock amps and one of them had a slight buzz that eminated from a transformer. The buzz was related to a quality control issue with the epoxy inside the transformer in a small batch of transformers they had received right around the time my amps were built. If the transformer didn't develop a buzz within a couple of weeks, I was told it never would. The other amp was perfectly silent, by the way.

Based on my experience, I would ask your shop to return the amp to the manufacturer diagnosis and replacement of the transformer if necessary.

I have had my amps for over a year and both remain silent.

Absolutely return the unit.