I just bought a very clean well maintained 2005 E320 CDI from the original owner and I am in CA so I actually had to use the A/C for the first time yesterday and while I was cruising down the freeway I noticed a hissing sound that seemed like it was coming from the dash behind the gauge cluster. The sound would come and go as I accelerated and decelerated. It is very annoying and quite loud. When I turn the A/C off is seems to go away. Anyone experience this and what was the fix?

Could be your fan making noise but it's behind the glove box. Mine made different levels of sound depending on if I was accelerating or turning. There is a thread somewhere on how to take out fan and lube. I've done it before and it's working perfect. Not sure if this is your exact issue but it might be.

Is it a sibilant hissing sound coming from your center AC vent? You could be low on AC refrigerant. My W210 did this for a long time before I knew what the issue was. As refrigerant circulates through the system, it should change state from a liquid to a gas when it passes through the expansion valve. If you are low on refrigerant, it may be changing state before it gets to the expansion valve and thus you heard the sound of gas moving through the EV. IIRC the EPA has determined an up to 10% loss on a yearly basis is acceptable for an AC unit, so while I would be horrified to lose that much at a time, your car may be operating normally and only need the refrigerant topped off.

I am pretty sure it isn't the Freon being low because the hissing is still there even if I turn the a/c off but leave the fan on. as soon as I turn the fan off the hissing goes away completely. The hissing fades in and out as I am driving and seems to be louder and more apparent when I am going up a hill and the engine is laboring or when I am accelerating. The A/C is very cold. Annoying as heck!

Hi there, my W210 came across the similar situation. I have no idea where the hissing sound coming from, I checked all the speakers - hissing noise particularly foot on/off accelerating pedal - which is very irritating. It was gone until my AC suddenly went into defrost mode. It is due to a vacuum tube connecting between the firewall and the intake manifold.

Having watched this video , I figured out something.

@4:11 he explained - the vacuum tube connecting to intake manifold which keep the AC running normal. If broken/disconnected, the AC turn into defrost mode which make your screen blurred all the time and there's no hissing noise.

I bought a new vacuum tube and reconnected them together - AC turns normal but hissing noise with your driving which is irritating.

@0:40 he explained - there's vacuum reservoir box - which my car don't have this - which means the hole keep idle at the PSE pump at (top 2nd left - no tube, no connection). I have yet resolved the problem, but I'm sure the noise is relating to the PSE pump and AC.

Good day! I came across this old thread today April 16, 2022, because my 2012 C300 W204 just started making the exact same sound from behind the behind the center of the dashboard. Did you ever find the cause of the problem?

2 years since your post, did you ever find out the cause and fix for this problem?