I would just check the vacuum connections first and see if something came undone. That's what I thought of when you said "hissing noises". I know many on here recommend the miti-vac or something like that. I haven't used one yet, so maybe somebody will chime in.
1994 C 280 117.5k, White (Good as new)
1997 Toyota Camry 149k Miles (Not so pretty anymore)
1990 190e 2.6 95k (Sold-Should not have)
1981 240d Stick ??? Miles...sold