Sounds like an engine surging. 3 regular 'cannon shots'. Notoriously non directional sound, but very strong. It's not much more than a loud 'cough' and usually self recovers quite happily. It's caused by an excessive pressure mismatch through the engine.
The VC10 Conway engines used to surge merrily at altitude given the slightest excuse. I was once occupying the Engineers seat whilst he saw to his 'ablutions'. We were given a climb, and I opened up the throttles what I thought was 'gently'. The engines gave 2 loud bangs, followed about 3 seconds later by a lanky Scottish Flight Engineer bursting onto the flight deck with his trousers around his knees swearing at me what I know not- I don't speak Celtic, but I think I've heard it at Rangers/Celtic football matches. It was also guaranteed to shatter knicker elastic of the cabin crew working in the rear galley right next to the engine- apparently it was a sound beyond human comprehension. But quite harmless.
I was also walking across a car park at one of the Heathrow hotels under 28R lift off end when a Pan Am 747 surged just after lift off right next to me. I can confirm the destructive effect on underwear elastic. I'll swear the whole aeroplane shuddered and shook, but neither I nor the people within were probably actually too observant at that particular moment.
I was also in a 707 cabin in South America right next to the engine when the aircraft landed and lost all hydraulics. The only way to come to a complete halt and stay there was by keeping the engine in reverse. It surged continually until somebody came out with chocks, and in the evening twilight, it was spectacular. Big sparks flying 20 feet, explosions every few seconds, incredible racket, smoke. It was the most fascnating thing I have seen in aviation, tremendously entertaining.
I would assume the XLs were 757s. The 737 engines are quite boringly stable. the RB211s of 757s can have their moments.