Mine has been doing the same thing.
Last cold snap it would hiss every once in awhile, but went away as the weather warmed up. Now its always hissing.
New brake booster from RockAuto was delivered today, hoping to swap it this weekend. It seems like people with '99s have this problem more often than other years, not sure if thats because there are more 99s, have more issues, or...?
The hissing is the #1 characteristic of a blown, or blowing booster. Supposed to be a pretty easy job, about an hour of work due to some inadvertent work done by Chrysler engineers.