hey guy's i've got a Laney LX120 RT Twin, had it nearly 2 years its been the best thing i ever bought. Until yesterday.

I had been playing guitar earlier in the day, i came back to play some more and turned my amp on. Pretty normal so far...
But then, shock, horror! I started getting a terrible, loud feedback noise, so i did the sensible thing and turned my guitar volume off but it didnt work and the noise continued until i turned my amp off! :@
i came back to it today and even with nothing plugged into it the noise still sounds, which should be impossible as there is no sound going in to be 'fed back'
anyone out there had this problem or know what to do? it would be much appreciated ^_^ thanks!
You might have a microphonic preamp tube. Assuming this is a Tube amp. If you have an extra preamp tube, use the process of elimination and pull one out at a time and replace it with the known good preamp tube. And work your way through the preamp section and see if this fixes the problem.
If you start a reply with: I have never played one but I have heard good things about it! Your opinion is invalid.