Noise when letting off brakes

[jesc]

I just bled the brakes and got them working fine. No abnormal sounds at all. Later, I turn on the car for a moment, turn it off, and now the brakes are making an airy sound when I let off the brakes. There is no sound when depressing the brakes. The sound kinda sounds like gurgling, or maybe when you're sucking the straw from a cup of soda you just finished. The sound comes from around the master cylinder/brake booster area, but I'm not sure which component for sure since they are right next to each other.

I made sure there was no air in the master cylinder by cracking open a metal line and letting it weep fluid. I'm suspecting a brake booster vacuum leak. Has anyone had this problem before?