Originally Posted by
inbalance
I doubt that you heard them from 15 miles.
You probably heard the other two chinooks, 4 miles away, with their transponders swiched to stby and not showing on ADSB.
Four miles away would have been rather more audible and I would probably have seen them! During lockdown when there was virtually no traffic about and it really was quiet up on the common I could hear a Lynx following the Thames heli lane. In the right conditions, noise can reach sensitive ears a long way...