Quote Originally Posted by JohnnyEgo View Post
The fluid surging sound combined with no apparent issues with brake fade or squishy pedal feel makes me think it isn't brakes that are your issue. Brake fluid isn't very high volume compared to other fluids, and there isn't much space for it to even make surging sounds outside of the master cylinder and reservoir. If you had fluid low enough to make a surging sound in the master cylinder, I suspect you'd experience brake issues. Having said that, squishy feel is a sign of brake issues, usually air in the system.

Assuming it isn't the brakes, I'd consider what else might have fluid in it and be stressed under braking. Condensate drain tube blockage or air in the radiator or heater core circuit will cause a sloshy sound that you'd likely notice when braking.
Agreed. Sloshing liquid won't be a brake fluid issue.

Without hearing the groaning it's impossible to diagnose. Your suspension components can make odd noises under braking because of weight transfer but are usually not a long sound.

Try to record the noise. Or drive by a shop. Ask one of the guys to step out for a second and have him listen as you pass. It is often easier to pinpoint a noise from outside the car.