Screws used for height adjustment are usually made of a ferrous material that is plated with nickel, chrome or gold. The height adjust screw used on instruments such as Fender Stratocasters, Telecasters, Gibson humbuckers etc. can attract the magnetic field from the pole pieces or bar magnet and can act like a pole piece. The height adjust screws on a Stratocaster can draw the magnetic field from the end pole pieces in the pickup. If the height adjust screws are loose, they can vibrate and cause feedback. Make sure the Fender style pickups have a rubber tubing for the height adjustment to keep the pickups firm within the pickguard or bridge plates. I often wrap rubberbands around the height adjust springs on humbuckers to help eliminate unwanted vibration or feedback from the height adjust screw and compression springs.