Hmm - you'd never have guessed actually unless you remembered another post I did about something I broke recently...
Didn't think much of it on my way there - had music on as usual and wasn't really listening for it.
Inde couldn't reproduce it once I was there. Surprising but I said we can try as I knew how to recreate it.
Nothing. WTF?
I worked it out in the end - I broke my mirror and bought a new outer casing - but needed a bracket fabricating to replace the bit I broke off on the frame (to save £300 for a Merc frame, or £120 for ECP one).
Meanwhile, I had used sticky tape to hold the old casing on and it had been fine for a long time - or I'd never heard the sound as I usually have music on.
Then a couple of weeks ago (when the noise started coincidentally), we took it off so our engineering bod could do a reccy on what was required - taped it back up as before.
Noise started but I only heard it on a subsequent morning - so it didn't correlate exactly and the noise sounded like the rear NS.
Our engineering bod finished rustling up something that fitted perfectly - and yesterday we fitted the new outer casing - which went on perfectly.
I had music on on the way home yesterday - so unlikely I would have heard the sound anyway but when I couldn't hear it this morning...
FFS.
It seems I was lucky with the first taping up - as both times since it has made the noise - and because the noise was actually coming from the front, that might be why my inde thought engine mounts!
PS: I should have said back left or NS corner in my first post - I typed right probably because it occurred when steering right.