2003 Honda Accord makes noise like leaking balloon when braking

Hi,

I bought a 2003 Honda Accord three months ago with about 190000 miles on it. I have since put at least a couple thousand more miles on it, but it has been driving without any problems until today.

When I pulled out of the parking lot today, I noticed a loud noise coming from the back of my car, to the right. It sounds like a deflating balloon (like when you pull the lip of the balloon closed and it makes that PBBBBT noise) and is very loud. I want to say that it has only started today, but I usually have music on so it's possible that it may have been happening before, just more quietly.

It only happens when the brake is engaged, and then only when it is about 2/3 of the way depressed. It increases in loudness as the brake is further depressed, then fades away as the car comes to a complete stop. When the brake is fully released, the sound happens again, but more quietly and more like a whine, then fades after about 30 seconds to a minute.

Weirdly, the sound doesn't happen when I'm in reverse even when the brakes are engaged.

My brake does seem softer than when I first got the car, but I am still able to slow to a stop within about 100 metres by carefully pumping the brake without causing the sound to start. The function of the car doesn't seem otherwise affected, and I have not noticed any particularly pronounced list in direction when I stop (though I might not have noticed, given the quality of the roads where I work).

This is my first car, so I'm not very experienced with it and don't want to self-diagnose the problem incorrectly. Please help! What could it be?

Thanks!