tribute whine

2001 tribute with about 100K Kms. Recently, when driving, sometimes we hear a whine that goes up in frequency (to about 3000Hz--wild guess!). This whine stops suddenly when I touch the brakes. There is no discernible change in the efficiency of the performance of the vechicle. On a 5 minute drive last week, it happened about 5-10 times. Each time it stopped when I touched the brake pedal, but recurred moments later. I haven't heard it since. Any ideas?