Vibration noise from toroid transformer. Is it possible to get rid of it?

My homemade amplifier (D.Walton 300B XLS MKIII schematic) is working normally with one exception. When I turn power on the amplifier  for the first 30-45 seconds appears vibration like silent but bored noise and this leads me crazy.  Others suggest not paying attention to this but I can't.

Maybe not quite well, I covered two main toroidal transformers under one cover and sealed it with epoxy.

My attempts to find the source of the noise were as follows:

I grounded inputs and verified voltages – everything – OK.

1. Then I removed all the tubes, turned power on – from the main transformers cover - dead silence - everything is OK. No load - no noise.

2. OK - I inserted GZ37 only, turned power on –  after about 10 - 15 sec appeared growing vibrating noise from the main transformer, which after 30 seconds became poorly heard.

3. Then I  disconnected one of toroidal transformers from the main. What was interesting I found high voltages in completely from the main disconnected second transformer, only not standard ones, but about 300 – 0 - 500 V.  When connected again both transformrs, voltages also have become standart 450 - 0 – 450 V on both transformers.

Maybe this electromagnetic interaction between two closely fitting toroidal transformers under one cover is the cause of noise? But I can’t do anything because these transformers are already deadly buried in epoxy. Replacing them with new ones and making anew different layout would cost me over 400 € and almost all of the amplifier to be rebuilt. Is it possible to change something not so painfully? Maybe the vibrational noise reason is different?

Kindly Antanas