I used to get flown around a lot in helicopters when I was serving in Northern Ireland. That noise always signified us going away from base or arriving. When in base not on patrols ets we could say with some degree of accuracy who was coming in and out. Different noises for other platforms etc. We had Wessex, Lynx, Puma and the odd Chinook for company troop movement.
I know this doesn't help but just a point to make of interest.