Brake booster?

When I depress the brake pedal it make a sound like a muted horn. The sound comes from under the dash. The pedal goes almost to the floor, but can be pumped up. No lose of braking. I did replace the master cylinder because the brake fluid looked aerated. The problem was there before the replacement of master cylinder.

This is not normal brake sounds like rubbing/grind of brake pads.

Could this be a bad Power Brake Booster?