Just FYI, I've seen this issue more than once, on a number of different makes and models. It'* always ended up being either a bad rotor surface (cut the rotor and it'* fixed), or bad pad surface (remove the pad, take some emery cloth and scuff the pad, reinstall). It'* very probable that the noise you describe will come back. If it does, scuff the pad, cut the rotor, and you should be set.
EDIT: Just wanted to clarify something -- it doesn't have to be emery cloth, sandpaper or something of the like will work just fine.
'94 Bonneville SLE | 266K miles
CAI - K&N cone filter - polished intake runners - 140 amp alternator - 2.5" exhaust - Magnaflow hi-flow cat - Magnaflow dual outlet muffler - 106K on engine and transmission - Sold 1/14/07
94 VW Jetta - 151K - 2.0 - 5 speed - Ghetto black steelie wheel mod - Techtonics 2.25" cat-back system