My replacement Class 5 developed the rattle/resonance problem and was really bad on bottom E and an octave higher.The noise would cease if i placed my hand on the upper area of the back panel.Looking inside the amp, the four cables running from the transformer to the board touch the back panel just where the noise seemed centered.I re-routed two of the cables under the earth wire that connects to the chassis.This pulled all four cables away from the back panel.
Put it all back together and the back panel can be felt to vibrate on certain notes but the noise is a tiny tiny fraction of what it was and is not a problem when playing.
Thanks to ClubAndCountry for his comments in the inside pix thread for the idea of trying this