Ok, this is weird - it seems related to the source material. If I play the Spotify track or my MP3 ripped track, the mechanical resonant buzz is there, particularly in the right speaker, and it's emanating from the cabinet, not from the driver. Again, the rear terminal panel and rear panel are tight. If I play the CD, either the buzz is not there at all, or it's at an attenuated level.
I understand that a ripped track would modify the waveforms, but it seems strange that a ripped version could set up a mechanical resonance, but the CD track wouldn't.
I haven't noticed it on any other track. It must be that one particular bass note...