Thanks for response. The sound was in fact emitted from one only 300b. When I realised, on reversing the valves, it was located in only one, I informed Hotrox and was advised this was not unusual and as the valves were fine on playing music there was probably nothing to worry about. Unfortunately, after a few days, the sound became prolonged and aggressive, probably a danger to a transformer. The valve involved was a new, three weeks old, Psvane black bottle /2. I am awaiting replacements. I had had no sound at all from my Electro Harmonix or my EH gold grids.
Thanks again for help.