My AC works beautifully now. From time to time while driving it seems to make a hissing noise. Maybe five seconds then it stops. It may happen again five minutes later. Is this ok?

monaco,

The noise you hear in my opinion is normal. It usually is most audible from the center dash vents. The noise is due to the refrigerant boiling as it initially flows through the relatively warm heat exchanger when the compressor cylcles on. The heat exchanger is where the air coming out of the center (and all the other air flow passages, but the path to the center vent is most direct) vent just dropped its moisture and heat, further encouraging the phase change in the refrigerant.

When the compressor cylcles off, the flow of refrigerant is interrupted, the remaining liquid boils off at a slower and quieter rate, cooling and drying the air on its way to the cabin. In this state, the heat exchanger gets pretty close to ambient temperature before the compressor cylces back on. So the first few seconds of new, liquid refrigerant that enters the heat exchanger when the compressor cycles back on, immediately starts to boil, and you hear the hissing noise which is transferred to the air passing through the other side of the heat exchanger membranes (one side has the refrigerant, the other has the air being cooled, and the boiling causes the membrane of metal separating these fluids to vibrate, and that vibration is transmitted by the air passing through the heat exchanger).

I hear it in all my MB's with functioning air conditioning. Hope this helps, and good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

My 300SDL makes the same "hissing" noise coming from inside the dash from time to time when the AC is on, I consider it normal, something to do with the evaporator in the dash.

Yeah, the airconditioning trade technical terminology for the heat exchanger where the liquid turns to gas is "evaporator" as opposed to the place where the opposite happens, which is called the "condensor." In regular terms these things are heat exchangers. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)