Solution 1: The units may be too close to each other. Make sure the parent unit and the baby unit are at least 1 metre/3 feet away from each other.
Solution 2: The volume of the parent unit may be set too high. Decrease the volume of the parent unit.
Solution 3: Only for DECT monitors: The parent unit may be too close to another DECT system or another baby phone that is disturbing the frequency.