Our 2000 4runner just started making a noise a few days ago. It’s not very noticeable unless you roll down the window. It is high pitched, and when it happens it’s intermittent, never a longer tone than a quarter second, rather like a bird singing in fact. It seems to stop while I’m touching the brakes, no matter what speed I’m going or how softly I touch the brakes. It seems to mostly happen when I’m gassing or idling. Coasting at higher speeds sometimes produces the sound, sometimes doesn't. It doesn’t happen while in park even if I gas it. Happens when the AC is on or off.
What’s making that noise, or how do I figure that out?