A hissing noise suggests an air leak in your power brake booster. Since the booster is right up against the firewall it could sound like it was coming from inside the car.

This may sound far fetched but bear with me.  There is a rod connected to the brake pedal that goes through the firewall to the brake booster and master cylinder.  It is usually covered in some type of rubber bellows and maybe a few squirts of WD40 will lubricate the rod so it doesn't rub and make noise when moving through the bellows.  It's worth a try.

Hissing when you press on the brake is relatively normal. The booster uses engine vacuum against a diaphragm to assist your foot. If there's a constant sucking sound or hiss, whether or not you're pressing on the brakes, there's a problem with the booster. If you hear a hiss, it should only be the moment you press on the pedal. 

My 1999 Ranger hisses when I press on the brake, so does my '79 Pontiac. I've also heard my 2017 Mustang hiss when getting on the brakes, but they only hiss when you initially apply the pedal. There is no continuous noise. 

Inspect the pad and rotors. Cheap brake pads, bad rotors can do that. 

Have you checked brake pads? Perhaps they are wearing out.

https://www.youtube.com/watch?v=0L0FR38YF2k

https://www.youtube.com/watch?v=dSKpnpPnQVQ

Thanks guys.  It was the brake servo (vacuum servo). My mechanic didn't bother replacing it as he was able to repair it. Hissing sound stopped. Thanks