I bought this amp a few weeks ago knowing that it had a problem so i got some money off, the seller didnt really know what was wrong im guessing maybe just a tube has gone but im not experienced in the tube amp department.
The problem is that it sometimes whistles pretty much randomly when playing but not all the time, i think it maybe when i play higher notes, there are a built in reverb and tremolo but i dont think they are the problem. Iv taken the back off the amp as i heard that by lightly tapping the tubes with a pencil or chop stick and listening for a 'ding' means that the tube needs replacing. But when i opened the back i didnt see any tubes and didnt want to remove the circuit board, is it safe to remove it end when isolated? Or should i send it to the music shop? And has anybody else experienced this with thier tube amps
Whistling could be a microphonic tube, or it could be a filter cap going bad.

The tubes in an AC15 are way deep in the chassis, you can't get at them like you can with other amps. If you don't know what you are doing it is not safe to pull the chassis to get at the tubes.

Luckily, learning what's safe isn't too hard - if you only touch the parts that you could touch before you pulled the chassis, and just the tubes, you'll be fine.