Dear Car Talk:
I have a 2004 Subaru Outback, 6-cylinder. It makes a high-pitched humming noise, like millions of crickets, after I drive at freeway speed, but only after about 10 minutes on the road.
It stops when I brake, then resumes. Oh, the Outback does have 272,000 miles on it. -- Joy
Congratulations, Joy. It's nice to see you marking the 50th anniversary of the moon landing by driving this car the equivalent of to the moon and part way back. Unfortunately, all you've got to show for it is a brake pad noise.
It sounds like you have a brake pad that's sticking, causing it to rub against the disc rotor. Normally, the brake pads sit right against the disc rotors, and even touch a little bit, but not enough to slow down the car, or make any noise. Then, when you press the brake pedal, the brake caliper causes the pads to squeeze the spinning disc rotor, which is what stops the car.
Based on your description, it sounds like one of your calipers is sticky. So when you first start driving the car, everything is OK. But after about 10 minutes (and, more importantly, several applications of the brakes), the caliper fails to retract all the way, and leaves a pad pushed up against the rotor.
That's what's making the sound of a million crickets -- the pad continually rubbing against the disc rotor as the wheel turns. When you actually use the brakes, and the pads are pushed tightly against the rotors, the noise temporarily goes away.
You should get this fixed, Joy. It'll eventually get worse. The danger is that if your brakes are always lightly applied, you can overheat the brake fluid. And if your brake fluid overheats and boils, it can't transmit hydraulic pressure, and your brakes won't work.
Ask your mechanic to check your brakes. In particular, ask them to check for a sticky caliper. But when you go, bring some smelling salts with you. On a car this age, especially if the brakes have been neglected for a long time, you could easily end up needing $1,000 worth of brake work. You could need pads, rotors and a caliper rebuild.
But if that's what it takes to make the car safe, do it, Joy.