:Electrolytic hum (or lack of electrolytic function due to their drying up) is loud at low volume, but does change as the volume is turned up. When the volume is turned up, if the amplifier is working at all with this problem, it will be amplifying the hum in the RF sections (the power supply feeds all sections of the radio, and if it is not filtered properly, all sections of the radio will have their own hum).
:
:Thus it is entirely possible that your electrolytics have dried up. If you use the radio frequently, and it suddenly started to hum, it is also possible that something has disconnected or something else has failed, but it is entirely possible that one night the electroltyics decided to leak out and dry up (especially if a seal went while the radio was still hot from previous use). I find it more believable if you hadn't used the radio for a while and then found the electrolytics to be dried up. Both scenarious are possible. Give it all a check-up.
:
:Thomas