[G8BBZ]

Hi,
Backfeeding 6.3v AC into the heater winding on the vibrator transformer was a popular way of converting the R210 to mains operation in the past. John's Radio in in West Yorkshire used to sell R210's already modified this way - maybe you have one of these.
As the original heater supply was AC - admittedly at 100Hz or so rather than 50Hz - it would seem that the hum is not caused by the use of an external AC supply but because of something in the set.
As you have the vibrator, it may be worth temporarily applying 24v Dc to the power input so that the set operates off the vibrator system to see if the hum is still present. If it is then it would seem to confirm a problem in the set.
The solid state vibrators (like those from the 19 Set site) are very good and if you have a suitable source of 24v DC I would recommend getting one of those and operating the set as it was designed to be - after you have tracked down the source of the hum perhaps.