Right...
I've removed the grille and both handles, taken the speaker out and checked it over (looks fine) and put it back, tightening the screws. I've checked that no wires are touchng the cone, taken the jack plate off, checked it out and reattached it.
It's quite difficult to access the internals as you can only really get inside through the holes where the handles are (there is no removable back panel). It's also difficult to see much as there is some sort of white padding against the wooden sides.
The bizarre bit is that it still responds to certain frequencies (now a C instead of an Eb) and if you put the cab on its side it seems to stop, while if you sit it flat you get the noise.
Any further ideas...other than keeping it on its side?!