The switch might have gone component-heaven :(
Sounds like it's partially making contact when it shouldn't be (hence the dimmer light, and nasty tone)- probably from normal wear and tear. That said, i've not used that unit before, so it could be something else; that's my best guess though. I'd imagine that the switch is a fairly standard one, won't cost too much to replace, and if you're a dab hand with a soldering iron you'll have no issues :)