I cant download the file :(
Before the rest of this about audio transformers I should ask:
Are you sure it was the audio transformer? Many things on the power line can cause the mains transformer to make noise (for much the same reasons as audio transformers) at random times which is not as significant of a problem, and the fact that this changes from day to day dosnt tell you anything about the amp.
Anyways:
It is not uncommon of for gapped EI transformers to sing along with the music. I would say its pretty common, but very few people notice it above the din of their speakers ;)
When you hear it, its normally ~3Khz and up. No bass, ever. VERY little mids - about the same as you get from playing just a crossed over tweeter.
What causes it:
It is caused by magnetic forces moving in and out of the iron bits of the transformer. The magnetic forces actually squeeze and stretch the iron and this microscopic motion results in sound. In extreme cases where something is loose it gets worse. The fact that the transformers sing along with the music is not an indicator of quality, or lack of quality as the case may be - very nice transformers can sing along. If you are sure its coming from the audio transformer, and it suddenly changes I would be worried.
Regarding mains transformers:
Mains transformers can also make noises when certain problems are present on the power line. Any DC on the power line is known for causing this. One day the amp is dead quiet, the next it makes an audible humming sound coming directly from the amp but the speakers/headphones are still quiet, and the day after that all is right with the world again. Its very frustrating. Be happy that the 307A has an EI mains transformer - Toroidal transformers dont play nice with DC. If the noise is coming from the power transformer, and it goes away eventually, dont worry.
