Very often transformer hum can be cured by tightning or loosening the bolts that mount the transformer to the chassis.
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I have a 302B that has the same issue (a low "hmmmmm", and I bought it brand new from a local dealer early in 2002. Is this just a Jolida thing? I haven't been bothered by it enough to consider replacing it though. One thing that I did to mine was to have my dealer replace the coupling capacitors with BIG Kimber units, and this provided a more powerful sound, especially in the bass. It didn't make a whit of difference about the hum, but the sound is better. Of course, the tubes are all better, too. I would love to hear from other 302B owners about this "hmmmm" sound to see if it's a common affliction.