Found this thread after doing a number of searches.
I hear what must be the same thing on my Fuze v2 when pausing. It's not present, however, when Fade on Stop/Pause is disabled. It happens what seems like a second or two after the music stops. Could be right at the end of the fade out. Also happens at the start of resuming playback of a paused track.
Rockbox 3.7.1 and the IEMs being used are MEElectronics M11+, if that's relevant.