[font=trebuchet ms]Be careful about this - tubes are hot, and have lethal voltages on them!

Use a plastic chopstick, a light drumstick, or a large drinking straw to gently tap on the tubes, one at a time. If one of them is 'micrphonic', you will hear noises coming from the speaker. Do [u]NOT[/u] use anything heavy or metalic to tap the tubes, as you could damage the tubes and/or hurt yourself.

All tubes are very slightly microphonic, but my guess is that one of yours is excessive.[/font]