My hearing aids won't stop whistling!Updated a month ago

Several things can cause that.  Wrong dome size - either too big or too small; wax blockage in the ear canal, and high-frequency sound set too loud.
 
 1. If it's too large, the dome won't go into the ear canal where it belongs and may dead-end into an ear canal wall. 
 
 2. If the dome is too small, the sound will leak out of the ear causing it to whistle.  
 
 Try various domes until you find one that goes all of the way comfortably and without whistling.  
 
 3. If you have wax blocking your ear canal, the sound will reflect backward and cause whistling.
 
 4.  The high frequency setting is too strong.  Reduce the sound at 4K.