I hear a hissing that seems to come from the driverside door or perhaps within the speaker, but only when I take my foot off the brake (and it doesnt happen ALL the time). Any idea what this could be? Loss of brake pressure?

 

Brake booster is bad.
It is leaking from the Diaphram internally.

You will need to replace it.

 

What are the potential problems if not replaced immediately?

 

Check engine light.

If it goes out all the way it will be hard to stop.
To see how hard go to a secluded parking lot. Turn the car off while rolling and try to stop the car.Hit the pedal once to twice till it gets hard then try to stop.

 

I haven't found a single website that has OEM Brake Boosters for my car...Any price range you can give me from a dealer/mechanic?

 

About $400.00 for the Parts and about 1 1/2 to 2 hours labor.[:@](OUCH)