Explain unactuated disk brake noise?

[oldsubu]

I'm having a problem with my left front disk brake on my old 1994 Subaru 4WD Legacy sedan. The symptoms are a noise problem but not at all like the usual situation when one depresses the brake pedal. I am intermittently hearing a light to moderate shhhhhshhhhhshhh sound when the brakes are NOT actuated. In other words while the hydraulic fluid is not pressuring and the pads are supposed to be floating in light contact with disk rotor. The intermittent sound occurs about half the time and varies. Once it starts it tends to be continuous until I step firmly on the brake pedal. Then it will be fine for awhile until going over a bump that seems to jostle its position. The braking function itself is fine and well balanced between left and right brakes with no sounds at all. The brake fluid is near the top of the fill bottle and I've never had a leak.

I pulled off both front tires and the rotors do have the usual minor radial lines where the pad have been slowly grinding in uneveness. I can easily move the calipers/pads a little back and forth radially about the rotor so it is not simply clamping down too hard. In fact the left is identical to the right brake so. Not used to evaluating rotor smoothness so am unsure if it enough to have them turned or replaced. However both left and right front brake rotors and pad wear is about equal both inner and outer. Think the pads were replaced about a year and one half ago and look about 1/8 to 3/16 inch maybe thickness so are about due for replacement. And there is no foreign matter stuck in the mechanisms.

What I don't understand is what is the actual mechanism for the floating mechanical alignment of the calipers/pads about a rotor? Not something I've seen any online source explaining that. Looking at the brakes themselves, I didn't see anything that is involved. I've been guessing there might be something that needs a touch of lubricant and the problem may be the floating mechanism is stiff maybe as a result off to one side? Or maybe the rotor is uneven enough that just the normal light contact is causing this sound. Of course I will be taking it down to a brake shop but was hoping to get some advice here before I walk in and look clueless.

[somick]

I would not worry.

All brakes produce some sound. The thing is that our ears do not always hear it since it is above or below the frequencies threshold our ears can accept. Check out the pads shim. You may need some brake pad lubricant on the shim (not on the braking surface!!!).

When I start hearing this noise it is usually a sign for me to replace brake pads.

Sam

[zique]

Pull your rotor off and look place a straight edge across it. I would bet it is slightly warped causing it to come into contact with the pad occasionally. Do you feel a slight vibration in the pedal when you depress? That is a good sign as well that your rotor is warped and in need of replacement.

[297Kncountin']

My advice to you (and I repeat this to myself) is to think about the age/miles of your car.
These cars can go through amazing amounts of stress/wear with relatively no problems.
Then, when there is a problem, it's relatively a big deal.
I had the same kind of noise and it was due to a bad caliper on the wheel mentioned.
You need to look at new calipers and or rotors too.
The original rotors last a long time.
There have been whole discussions about Subaru parts vs aftermarket.
But I bought a Warner combo caliper loaded with pads and it worked well.
But eventually you have to bite the bullet.