Brake Moaning

Hi. I'm stumped.

I drive a 2001, Jeep Grand Cherokee Laredo 2WD. It's equipped with the standard 4.0L Inline 6, automatic transmission and has driven 104,000 miles. ABS is standard.

Here's my question. Recently my car began moaning when I apply the brakes. The rotors and pads are good and I replaced the master cyclinder just recently - the sound existed before and after the master cylinder was replaced. The car stops fine, I do not have to push the pedal harder than normal, nor are there any problems save for the moaning.

The moaning only occurs when I apply full brake power and so typically it only crops up when I am at a complete stop shifting between gears. When I depress the pedal, and as I am approaching as far as the pedal will travel the car lets out a moan - 3-4 seconds in length, almost a sigh. It is easily heard inside and outside the vehicle.

Judging by the location of the sound (in the footwell or behind the driver's side dash), I would imagine the brake booster is the source. However as I said, there is no degradation in the amount of pressure needed to stop the vehicle. The booster seems to be working fine, but it also seems the only likely source for this kind of sound.

I'm a poor student so I try to do most of my car work myself. What could this sound be, how can I economically fix it, and what will happen if I don't?

Thanks,
Brian