Hot Water Pipes in Multistory House
When installing PEX, there are clamps that are supposed to be used to attach it to framing. There are also gaskets for when the PEX passes through openings and such. These are nothing special other than made out of a smooth plastic to allow the expanding/contracting PEX to more smoothly slide.

Sounds like these weren't used, or a few are too tight, or maybe the PEX is just rubbing a little bit next to a structural element.

Is it something to worry about? Probably not.