Hissing sound from the A/C
 

2002 E320. A fantastic car. I just started to notice a hissing/scratching sound comming from the A/C vents when its on, The AC blows cold but the hissing/scatching sound is there. I took it to the shop and looked at it for no charge and said "I might have froze something in the unit" Others say that I am low on R13A

You may have a leak somewhere

The TXV will often hiss if the charge is somewhat low, however this is not a constant hiss; it is cyclical.

the txv?

Pull your AC actual values (1-8) and error codes and post them.

http://www.continentalimports.com/ser_ic40142.html

Thermal Expansion Valve

It is most likely low refrigerant level, but a restriction in the system can cause the same thing, which is insufficient refrigerant flow at the TXV.

The best way to tell which is the case is to evacuate the system and recharge by weight. If the pressures are then within specs, you're good to go. If not, you're not.