Have I damaged my amp?

Hi,
I have a Music Angel 300B amp which has been fine for sometime now. A friend visited to hear the new single driver speaker cabinets I've made, to compare these with my previous Kef's (the amp had been on for hours) I unplugged the Rt channel only and we had a listen for 30secs or so then I swapped back, the amp was still on. A few days later it started making loud rustling sounds 'under' the music (not volume dependent) on the right channel with some squeal when I tap the chassis, there is also quite a lot of noise when I power down from the Rt channel. I have tried swapping the valves with no change and have checked inside for loose connections 'bulging' capacitors etc and can see nothing wrong. So is this a probable cause for damage and what could it be? the problem is becoming noisier but only starts after the amp has warmed up an hour or more. My first thought is to replace the main capacitors under the transformer. I don't have a schematic for it but does anyone have any views on this.

Thanks,
Iain