Have you investigated the possibility that it's the noise itself? Perhaps at a high or low frequency that you are less aware of. Especially given your notes abo

Have you investigated the possibility that it's the noise itself? Perhaps at a high or low frequency that you are less aware of. Especially given your notes about the microwave and car rides. Those are both noisy environments, and both can have a regular mechanical sound at frequencies that may irritate you but you may not be paying much attention to. Try standing next to your AC unit or similar mechanical system when it's running and see if it also causes a headache.