it's likely coming from the tiny gaps around under your seat clamp

Since the beginning

System Weight Rating: 140kg (306 lbs)

With a large 60 kg capacity

Burgtec Seat Clamp - 39.7mm - Rhodium Silver PDP|PLP|Suits Freehub Type it's likely coming from theEffortlessly subtle and minimal design to keep your seat post secure and add the perfect last splash of colour to your build. No edge left unsmoothed, and no detail overlooked, even the split in the back has been machined wide enough so you don't need to detach your dropper cable which makes installation a breeze. A stainless steel bolt gives reliable clamping power and ensures no rust even in the grimmest conditions. Features Stainless Steel Hardware