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I can think of one thing. If that rotor is too thin. If it's been machined too thin in the past, that could cause problems with heat and thus possibly the rumble you hear. Although, I have no manual or specs for that vehicle, most of the time the minimum thickness is stamped right on the rotor. You will need a micrometer to measure the thickness, plus measure in 3 spots around the circumference to check for uneven thickness, which also could cause a rumble before it becomes so bad the brake pedal actually pulsates when braking.
Also, check that wheel bearing for being loose or worn.