I have this problem 2. Very annoying! Going to investigate now.

I once dropped my car into Ford to have this diagnosed - the mechanic suggested the exhaust mounts needed adjusting in some way.

I booked it in to have this done, in the meantime i removed the spare wheel from the boot to see if that was the source of the noise - and the noise stopped for a couple of days. So! I cancelled my booking, and you guessed it....the noise came back a day later.

Not been bothered to book it back in since lol, i just turn up my music now.