Why do hearing aids whistle?

[Golem]

I appreciate it's probably something to do with 'feedback', but often it seems to have no relation to the volume of background/foreground noise and just starts whistling. And how does the wearer tolerate it?

[Clanad]

It's caused, generally, by an ill fitting device. The amplfied sound from the aid enters the ear canal properly, but, due to a number of possibilities, the amplified sound leaks past the seal, exits the ear canal and re-enters the device resulting, as you've already determined, in feedback. Newer digital devices have practically eliminated the problem, especially the models designed to be worn fairly deeply in the ear canal...

[RATTER15]

As Clanad said, usually because they are not fitted correctly.

[Golem]

Thankyou Clanad and Ratter15 for your replies.

And thankyou Clanad for your answer to another of my questions some weeks ago that I neglected to respond to.

[Clanad]

De nada, Golem, haven't seen you here in a long while, welcome back!

[tubeway]

It can be because of an ill fitting device - or of the vent holes are too big. As in the previous answer modern devices with dynamic feedback cancellation are much better at reducing feedback but can not eliminate it entirely.
As the feedback is a high frequency sound, many wearers with high frequency losses - will not hear it as well as the people around them.
As a hearing aid audiologist feedback and occlusion (the other side of the same coin) are major problems for many people