I have a hearing aid and recently I went to my local hospital for advice as the device was whistling quite often. The technician explained how the sound would bounce back from, eg wax in the ear and cause the high-pitched sound. He took the device away to check it and as I gazed around the office I noticed on the wall a sign extolling the various services on offer but was astounded to see at the bottom of the page, highlighted and in bold letters:

****** FEEDBACK IS APPRECIATED *******