The reason that hearing aids can begin making a whistling noise is due to feedback which usually means the hearing aids are either not working properly or need to be adjusted to avoid the feedback from the hearing aids.
If your hearing aid is making a whistling sound it's caused by feedback that means that the sound leaving your ears goes back into the microphone of the hearing aids resulting in the feedback or whistling sound.
Sometimes the hearing aids just need adjustment to fix the problem.
I would take the hearing aids to a hearing aid or medical supply place or where you got the hearing aids and have them see if they can adjust them.
Let them or your doctor know about the whistling sound because the hearing aids should not be getting feedback and making the whistling sound.