Welp, I know we're all really interested in this, so here's an update.
Did some research (youtube) and found some steps to take to see if there is a vacuum leak in the brake booster or somewhere between it and the brake pedal.
Ran the car for a bit, pulled into the garage. Put it in park, foot off the brake, turn car off. Hissing noise tapered down for about 5 seconds to nothing. Wait 2 minutes, then test.
The test is to press the pedal down with the car off for 2 or more minutes. If there is a leak, the vacuum will have leaked and you'll feel little to no assist pushing the pedal. If the vacuum is good, you'll get 2 assisted pedal pushes.
I got 2 pushes no problem, even got some assistance on the 3rd push. So it seems there's not really a leak there.
Any thoughts as to what can make a hissing noise down there that stops when pushing the brake pedal in? I actually stuck my head down there while it was running and it seem like the noise is coming from behind/within that little rubber plunger. What is that?