The Noise detector will notify you when the recording is "Unreadable" because it could not be read as an EKG, likely due to signal noise and/or incorrect recording technique. This can be caused by signal noise from physical movement, dry skin, electrical interference or other sources. To try to solve "Unreadable" results, try to relax and hold still, rest your arms or move to a quiet location.”
Read the article What can I do to avoid the Unreadable results? to learn some strategies to avoid Unreadable results.