The background noise is very likely a signal being sent from the Echo Dot. A good way for you to find out is to connect another Bluetooth device to the ADAPT, and then try stopping the audio. If the ADAPT generates background noise, the problem is in the ADAPT. But if it is clean and the noise only happens when you stop the Echo Dot, this will confirm that it is the Dot sending the noisy signal.
If you have concluded that the Adapt is indeed the one causing the noise, we would like to work with you directly to help you with your issue. Please submit a case request at https://www.harmankardon.com/support-contact.html and the Customer Service Team will be more than happy to assist you.
Date published: 2018-01-15