I live in Blackheath in London which is in the borough of Greenwich and Woolwich, and Woolwich has an army barracks. I assume that’s where most of the Chinook helicopters come from (or visit and then leave) that make lots of lovely noise overhead in the early evening. With two rotors (spinning in opposite [contra-rotating tandem rotors] directions) the Boeing Chinook RAF helicopter has an unmistakeable sound which you can hear well before you’ll see any sign of a helicopter. They also look totally cool when they do show up.
Here is a low quality (sorry!) iPhone 5s sound recording, taken without wind guard, of a Chinook making its lovely noise in Greenwich.