I have created your post as a new topic as this is a different problem. Some chargers have been known to cause this issue on the FP (and some other models of phones from Google searches). It is usually caused by unshielded chargers causing radio interference or from placing your phone on a surface that may make the issue worse. One guy reported on XDA that placing his phone on his bed caused this issue which was probably due to RF interference amplifies by the metal springs of the bed
Usually trying a different charger or cable will resolve the issue. Also charging from a computer usually doesn’t cause this issue 