Audio Research Refernce 75se

De-lamination of the transformer is very likely. I owned a Counterpoint 5000 preamp years ago that started to develop a mechanical buzz that didn’t come through the speakers. It turned  out that this was a known problem with this transformer. And it was not a cheap Chinese unit, but a high end made in USA transformer. Sh_t  happens!

my ref 75 is silent

@discopants 

Thanks for clarification. These are led lights indeed, not halogen as I mistakenly said and the dimming part is causing the amp buzzing. Without dimming them, zero buzz.

The threat of excessive repair costs is why I build my own audio whenever reasonably possible to match or exceed far more expensive factory made amplifiers and preamps using the same parts and design. Often you can use better parts than the manufacturers use. If something goes bad you can easily identify it and replace it, but more than that, you can always use heavier duty parts rated to several times the current and power at which they will operate. 

You do not always get what you pay for. One $10,000 tube amp uses electrolytic filter capacitors in the power supply, For less of a price they could afford polypropylene capacitors which have a better ESR and do not go bad after a few years of limited shelf life.