vtl pre amp

Hi all, I have a pre amp with a problem,I had a scratching sound coming from the right chanel, I changed the tubes over to left side and left side had the scratching sound. I was told that thier was a bad tube, so I replaced all the tubes with new ones .The problem came back, scratching in one then it seamed to go too both chanels I have to be quick because the speaker woofer goes crazy, I sent it away to a tech, telling him all my problems with the pre, he has had it for 2 weeks now and can't find the problem, he has it on top of the amplifier so it is getting plenty hot too.{HELP} has any one got an idea? I have my old pre amp in my system at the moment with know problems.
well did happen to me with one channel scratching after i change to diff tube.i did send to repair only minor.socket holder got loose.he just solder..