It's growling at me!

I just did a brake fluid flush on my car (2002). This was (what I think) the easiest first step into why my brake pedal hits the floor. The brakes are still effective. The pads and shoes still have more than half the material on them. They will lock up the wheels and the ABS seems to work, but the fact that the wheels lock about the time the pedal hits the floor makes me a little uneasy. There is a SLIGHT spongy feeling to the pedal, but it doesn't feel like a leaking/bypassing master cylinder usually does. After the fluid exchange operation was done, the brakes felt and acted exactly the same, BUT....

After all 4 corners were bled and all the old fluid flushed out, my dad was next me while I was topping off the reservoir. The engine kicked on and we heard a short growling noise, like a valve opened somewhere and was taking in air. But it was so brief that we had trouble pinning down the location. It would intermittently make this noise while the engine was running, but not with any discernible frequency and not with any other audible changes in engine noise. I don't remember hearing this before the fluid exchange, but I didn't really pay that much attention. We think the noise is coming from the area between the engine and inverter. And since we can hear it from behind the car, we're thinking it's low in the engine bay. Could this be WHY my brakes feel the way they do, or possibly have been caused by the exchange/bleeding process?

For bleeding, I found another post on here that (for Gen 1 at least) said it's done like any other car. I had my nephew operating the pedal, while I opened and closed each bleeder valve, all with the ignition in the RUN position. My dad monitored the fluid level, so it never took in any air. EDIT: DOT 4 fluid was used in this process