I have an odd problem with microphony...

Occasionally my system makes a hum that sounds almost like a musical note. Two days ago it started up and, as usual, when I switched the source switch it stopped but would start up again if I tapped the preamp or the phono pre. It seems to react more strongly to the pre than the phono but I will also hear a similar sound if I tap the TT. The most odd aspect is that yesterday I played the same record and there was no sign of this hum. I have a Lamm LP2 Deluxe and a Lamm LL2 Deluxe, both tube amps, and I recently added Herbie's Tube Dampers to try to cure the problem. Any ideas out there?