Squeal is usually associated with loose or vibrating parts within the pickup. Making sure the many ferrous components in the pickup are potted in wax help reduce unwanted vibrations that can lead to squeal. Single coil pickups are usually potted in wax to eliminate any minute vibration within the coil. Ferrous components near the coil and magnetic field can also cause squeal such as vibrating springs used for height adjustment in humbuckers or single coil pickups. Also height adjust screws that are loose can vibrate causing unwanted noise in the pickup.