Some guitars behave perfectly at low volume and then fall apart the moment the amp gets loud. That does not mean the player imagined it. It means the guitar needed to be tested closer to the way it is actually used.
At volume, parts vibrate differently. A loose tuner washer, pickup spring, jack plate, bridge screw or wire inside the control cavity can stay quiet during a quick check and then start singing with one note through an amp.
The useful clue is usually the condition that makes the problem happen: one pickup, one volume level, one room, one note, one cable angle, one playing attack. The repair starts there, not with random parts.