Some cheap (or expensive but old) pickups rattle when you shake them. Here's a tutorial on "repotting" some pickups in wax to fix this.

https://www.projectguitar.com/tutorials/electronics/repottin...

You don't have to be a cork sniffer to hear small things affect your tone. You just have to play very loudly on half-broken instruments - which isn't everyone's cup of tea.