Microphonic effect. It is in almost every in-ear headphones.
I found it got worse with the BeatsX the more secure I got the fit and pushed the buds in. Which I guess probably also happens with all in-ear headphones, but for running it shows that you need a balance between having a really good seal and just a good enough one.
I'm returning my BeatsX, they're just not for me, but I think they're a good step in the right direction for Beats/Apple. Some nice features and ideas but just not quite there yet. The cable arrangement and magnets in the buds worked well in the office but I didn't like them for running.
The sound quality is definitely better than the Airpods although I still think that's largely down to the better fit you get with in-ear buds which makes it hard to objectively compare them. The mid and high range sounded good to me though and there was a good depth to the sound when you got the fit right. The wingtips definitely help.
I do think if you use headphones when running, you've probably got some very specific requirements for headphones - its amazing the little features that annoy some but others love. I thought Powerbeats3 were almost perfect for running, but they keep breaking, so I've reverted to my £30 Sound Peats bluetooth headphones from Amazon.