Had the same issue with mine. Its a build up of dirt on the top speaker grill. As its waterproof you can wash in warm water or spray with contact cleaner also does it i found this via a forum search last year.
Im using a set of Soundcore Liberty Air with a set of Comply Foam tips and they dont move when im in the gym.
My others are BOSE QC35 II and Soundcore Liberty Pro as they also offer slightly different listening experiences.
Had a pair of Bose Soundsport and have to say they were disappointing...