It is normal for a hearing aid to whistle when it is held in your hand, or if you turn it on before putting it in your ear, or if you cover it with your hand while it’s in the ear. Once it is placed in your ear properly and at your preferred volume setting, it should not whistle. If it continues to whistle this could be caused by a variety of factors. Some things that can cause feedback (whistling) include excessive earwax build-up, the earmould not sitting in ear properly, split tubing and increasing the volume of the hearing aid - so check these first. If none of these factors apply, you may need to go back to your hearing specialist for further advice and possibly a new earmould or reshell.