Where do the whistling sound come from? Bird whistling or etc? (It was my hearing aid!)

In the Library at my hearing high school, my sister's friend kept asking me where the whistling sound comes from. I had to lie and gave her a terribly hard time. It was my hearing aid that whistled. It was covered by my thick hair due to my embarrassment. I wanted to fit into the group of hearing friends. -- My earmold was terribly bad and it got my ear infected!