Solution 1) Check the battery. Make sure the battery is fully charged after removing tab, before inserting in the hearing aid.
Solution 2) Check the wax guard for debris. The small white tips on many in-the-ear hearing aids are replaceable by the user. Locate the replacement wax guards in a grey or white package. Remove the old wax guard with the empty end of the replacement tool. Turn the tool around and snap the new white wax guard into the tip of the hearing aid.
Whistling, also known as feedback, occurs when sound that should be entering your ear is leaking back to the microphone.
Solution 1) Make sure the hearing aid is properly inserted into the ear canal.
Solution 2) See your audiologist. It is common for an accumulation of wax in your ear canal to reflect sound out of your ear back towards the microphone.