I have a1995 honda civic dx. It creates a few seconds noise like a low whistle at speeds between 30 and 40 mph or at slowing down at about 30mph. I went to a mechanic and couldn't figure out, brakes checked okay. This happens about 2 or 3 times at said speed from cold engine start.

often vacuum leaks, try this video https://www.youtube.com/watch?v=zMok2y05jNE