1999 Jeep Grand Cherokee Hissing Noise - goes away when bra

Just recently we started hearing a hissing noise (like air is leaking) when we drive the car. It's there all the time except when you depress the brake pedal (even just a little bit). My mechanic thinks it might be the rubber boot or seal in the power brake booster pump. I'm very mechanically inclined and was thinking about doing this work myself. Can you provide some insight on whether that maybe the problem, how complex the repair would be, would it require any special tools and can I just replace the rubber boot / seal or do I need to replace the whole pump? Thanks so much!

PS - any links on how to actually perform the repair might be helpful too.