Happened last year too but dd2 was sb at 34 wks so dh is worried in case there's some sort of connection (post mortem came back with nothing so we're extra paranoid at the moment, esp as I've just hit 29 wks).
None of the relevant professionals I'm dealing with seem to have any experience of this and haven't heard the click as I can't trigger it, unlike, say, getting bump to kick. Must add that dh has heard it too so I know I'm not going mad.
Thanks