I often get pickups in that look like they have been used to pickup debris from the bottom of the ocean. To loosen the ferrous debris you need to remove the magnetic field they the objects attract to. On humbuckers it is easy to remove the bar magnet and use compressed air to blow away the debris from the pole pieces and around the bobbin. You then need to use a fingernail brush or old tooth brush to remove the debris from the bar magnet. I’ve used masking tape or carpet tape to collect the debris from the bar magnet. I’ve had some pickups with non-removable magnets that were so bad that I demagnetized all the magnets to get rid of steel wool from all the nooks and cranny’s.