You have lots of miles so many things are possible. By your description it's quite possible it's the brake wear indicator puctured above. Those are used in many disc brakes and usually make a noise that comes and goes at first and annoying enough to get your attention.
Sometimes you can just look thru the wheel and see these things are close enough to touch the rotor or even show evidence on the rotor. I suggest a full brake inspection even if work has been done recently there's enough miles on this for some surprises,
T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/