It is caused by amplified sound from the hearing aid getting back to the micropphone and being re-amplified. Ways to avoid Whistling /noise Once the hearing aid is properly seated in your ear,the whistling will generally stop.

Hearing Aid Fitting: Behind the Ear (BTE)
Power Source: Rechargeable Battery
Items Included: Hearing Aid Case, Extra Selicon Head, Brush
Sound Processing: Digital
To Fit: Both Ears
Colour: Silver
Brand: Unbranded
Type: Hearing Aid
Department: Unisex Adults
Manufacturer Warranty: 5 Years
Labels & Certifications: CE Certificate
Features: Rechargeable