I am getting quite a hissing noise in what I assume is the evaporator - when the system cycles on and off - is that normal?

The system was converted to 134a before I bought it. I recently had a new condenser, rec/dryer and exp valve installed professionally. And it's cooling nicely.

Just wondering about the hissing...sort of annoying to my ears.

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DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School
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12 Ford Escape, sold, forgotten
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06 Passat 2.0T, PITA, sold

Las Vegas NV

From where do you hear the hissing noise? From the dashboard or engine department? When does it makes the noise during the on cycle or the off cycle? You have to specify better. The evaporator doesn't make noise. Yes the expansion valve can make a noise because it fluctuates inside and with some brands they make this noise.

The hissing is coming from inside. I can hear it as I drive. I would say it is probably coming from the evaporator as gas expands in it. The hissing lasts two or three seconds and starts a second after the compressor cycles on.

__________________
DS
2010 CL550 - Heaven help me but it's beautiful
87 300D a labor of love
11 GLK 350 So far, so good
08 E350 4matic, Love it.
99 E320 too rusted, sold
87 260E Donated to Newgate School
www.Newgateschool.org - check it out.
12 Ford Escape, sold, forgotten
87 300D, sold, what a mistake
06 Passat 2.0T, PITA, sold

Las Vegas NV

Such noises are somewhat normal for a system with an expansion valve. The 124 has the TXV mounted more or less directly behind the tachometer, so the noise is perhaps more audible than on other models of car.

My experience is the sound becomes much louder when the car is low on refrigerant charge. Since the system was just serviced, do you know how much R134 was used to charge it? As well, perhaps the new expansion valve is simply more vocal than the old one, and it's a feature, not a bug...

^^^This. Our car is the same.

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1998 W210 diesel (wiped out by a texter)
Baum spring compressor "for rent"

Hissing to me could potentially be vacuum. On my 95 e320 I hear a hissing every so often its the AC running. I'll listen more closely and see if I can't pinpoint it. I just assumed it was a vacuum leak that was "recharging" with additional vacuum once it got too low.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg