I have a 5 yr. old SVT-CL. Recently it has been producing a slight noise .... almost sounds like something rattling in the speaker cab ... that only occurs when an "A" note is played. Open A or 1 octave up is most noticable. It isn't loud enough to be heard at all when playing with the rest of the band, even at very low volume. Anybody run into this before? The cabinet is fine, the ohm setting is matched. Is it pre-amp, driver or power tubes? I would sure prefer a quick fix instead of the repair shop or all new tubes!!