Is the feedback only when inserting the receiver into your ear? That’s not unusual. If they aren’t already turned on you can have the aids set to play a startup tone where they beep 10 times or so. While the startup is playing you can insert the aid in your ear without getting any feedback.
The startup tone starts when you remove the aid from the charger. If you wait too long to insert it in your ear you may get feedback, as the startup beeps complete and the aid becomes fully operational.