With all analogue monitors/wireless devices there is always a chance that eventually some interference may occur, this does not mean that your monitor is faulty.
It may be something in or around your home that you are unable to change such as neighbours WIFI etc. Have you tried using it in a friend or relatives home to see if the interference is the same or better?
To prevent interference please make sure that the batteries in the Parent Unit are fresh and fully charged for best reception. If you have not changed your batteries within the last 6 months it would be advisable to do so.
How to change the Channel on your Parent Unit & Nursery Unit:
Turn both Parent and Nursery unit ON:
• Select A, B or C on the button on the left hand side of the Parent Unit
• Select A, B or C on the wing on the left hand side of the Nursery Unit
NOTE: the channels MUST be the same on both the Parent Unit & Nursery Unit in order for your monitor to work and sound to be transmitted.