W203 ventilation hissing sound

[kash]

Hi guys,

I have a W203 which has a strange hissing sound coming from the ventilation system. I have tried to isolate it by closing the vents but no luck, it almost sound asthmatic.

The hissing is fairly constant even if the fan speed in increased or decreased but stops when the fan is turned off completely. It sound like there is a restriction in the air flow, maybe a filter somewhere?

Can anybody help with this as its driving me nuts.

 

[Sp!ke]

Does the sound come from the headlight adjuster switch area?

These are vacuum operated and if the switch is knackered or the vacuum hose loose, the switch will hiss independently of the fan or anything else.

 

[kash]

As I sit in the drivers seat the sound appears to come from behind the dashboard area, is this were the switch is located?

 

[television]

Does the headlamp height adjuster work, if it does not,, its possible that a pipe is off on the headlamp end.

 

[kash]

Tried the head lamp adjuster switch last night and it works ok. I still think its something to do with the ventillation system as when I turn the fan off completely, the sound stops, when the fan is on at any speed, the hissing sound returns.

Are there any other filters in the system apart from the one over the battery which might need changing?

 

[AMGBoy]

Could be the air con gas is getting low