I have a "chih-chih-chih-chih sound that seems to be coming from the wheel(s?) of my 2003 Impreza with 39,000 miles. It stops when I put on the brakes. One garage says I need new brakepads & it was the early warning signal noise, but my manual describes that as a scraping sound. This is high pitched & sounds like cicadas.
If you have plastic wheel covers, pull them off and see if the noise goes away. I had the same noise with my 98 Ford Windstar for years caused by the wheel covers. Otherwise check the front brake pads for wear.
The fact that the noise stops when you apply the brakes makes me think it’s the wear indicators, as your mechanic suggested. Checking the brake pad thickness is not difficult. Stop guessing and have someone check the pads.