What you're hearing is a microphonic pickup, but there's different ways the microphonic thing can come about. Basically, something (or things) is physically oscillating. It could be
- the coil wire
- any other metal part of the pickup structure - polepiece/s, base plate for example, or even the mounting springs if you have it spring mounted
- the entire pickup could be oscillating... though generally you need to be running a lot of gain, loud, and the suspension has to be quite loose too for this to be problematic
That last one is the easiest to eliminate as a problem by physically dampening the pickup with your fingers as it squeals. The others aren't an easy fix, and even potting post assembly can have limited benefits.