'89 Civic brake noise

The other day I started noticing a strange noise at low speeds and light pedal pressure; it's like a rythimic, back-and-forth scratching
or creaking sound. Under medium or firm braking it disappears. (Too hard to tell if it is happening at speeds above 20 MPH.) I replaced the front brake pads on my car a couple of months ago and the brakes are working fine, no hint of fade or pulling. It sounds like it might be coming from the passenger side, or at least is loudest on that side. Any idea what might be causing this and how to fix it?
Thanks Matt
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