My 330ci has this weird problem. A/C was cold on the driver's side, but warm on the passenger's side. And there was a hissing noise coming out from the middle vent. Thought my compressor was blown.
Went to Johnson's Automotive to get the car checked for any leaks and recharged with R134a refrigerant.
Problem is fixed. No more hissing noises, the A/C is cold on both sides.
Cost: $100 for everything (check for leaks + refill)