Terrible hum after ~60 seconds

Hi, I had finished my A15Mk2, worked perfectly! Then I implemented the footswitch option, switching worked perfectly and the amp worked fine too. Now after some time I have a problem. When I have switched on the standby switch, started playing which works fine,after about 30-60 seconds a terrible noise starts (and it's not just me playing :-)), a loud hum.
I was thinking that there could be a tube problem, but after swapping the ECC83' around, I still have the same behaviour.

Hs anybody experienced something like this? Ideas? I have measured the heater voltage, which is close to 6,9. I think it was closer to 6,3 when I finished the amp.

What I find strange is that it takes a while before the humming starts. I also here some "clicking" sounds in the speaker just before the hum starts.

BR /Mike

Hi again, I just noticed that when I let the amp cool down for 30 minutes, I can play some minutes before the hum starts. So it looks like the hum starts when the tubes are really warm. Maybe I should try to replace the tubes?