Hissing noise from dash-speed related

[big x]

I posted this on another forum without a response so would like to ask here:

I've got a hissing sound that appears to be coming from the center of the dash regardless whether the fan or aircon is on or off.No noise at idle but it gets louder at speed and the note changes when the automatic changes gear...sort of like a radio that's not tuned in.
I can't hear any manifold leaks looking under the hood and reving the engine.Is there anything in the dash that is rpm linked ? The noise does seem to be linked to throttle position when moving.
The car is a 93 740i which shares many parts with the E34 540i engine/heating etc.

thanx

adam

[Kalevera]

Adam, does it seem to come from under the driver's side of the dash? The gong can sometimes generate irratic noise.

Need to know more about the location and quality of the sound, though. "Dash" and "hissing" are vague.

best, whit

[genphreak]

Adam, does it seem to come from under the driver's side of the dash? The gong can sometimes generate irratic noise. Need to know more about the location and quality of the sound, though. "Dash" and "hissing" are vague. best, whit

Perhaps also check the ebox fan (could be dying or have somehting in it- ever dropped leaves into your ebox?? don't risk it- the fan is too damn expensive to replace) or the windscreen wiper Pressure Device motor- it increases the pressure on the driver's wiper arm
to ensure a perfect wipe at 10km or 200km/h. It winds up dependent on speed, if it gets water in it it corrodes and then makes funny noises that appear to be from the dash (but are actually from the plenum chamber under the windscreen).
Get to it early enough adn all it takes is a new gasket, clean out adn regrease, too late and the whole thing must be replaced (new from the stealer: so $$)

[mystical_wings]

Its more than likely a vaccume line from the climate control leaking

[big x]

Thanks for the replies.The sound appears to come from the middle of the dashboard and of a vacuum type so...
On the other hand I did find some shredded tissue paper behind the ebox fan outlet which I can only assume a mouse tried to build a nest there !! I removed the top plastic grill to pull it out but couldn't work out how to get access to the back.Note I'm in England so the car is RHD.I'll do so more poking in that area first.

adam

[genphreak]

Thanks for the replies.The sound appears to come from the middle of the dashboard and of a vacuum type so...
On the other hand I did find some shredded tissue paper behind the ebox fan outlet which I can only assume a mouse tried to build a nest there !! I removed the top plastic grill to pull it out but couldn't work out how to get access to the back.Note I'm in England so the car is RHD.I'll do so more poking in that area first. adam

I agree its more like a vaccum line if coming from teh middle, or a stepper motor if your vent flaps are operated by levers instead of buttons in the climate control panel. Check the other side of your ebox fan by pulling the cover and removing the motronic ECU (undo the main harness connector and 2x10mm nuts to lift it out and get access to the other side of the fan to check for debris- have a vacuum cleaner with spout on the end to make double sure). If its a vaccum line be sure to get to it as it represents a airleak in your engine. If your flaps have stepper motors (more likely), perhaps one clicking, in which case do a search for other posts about them... Nick

[Kalevera]

Perhaps also check the ebox fan (could be dying or have somehting in it- ever dropped leaves into your ebox?? don't risk it- the fan is too damn expensive to replace) or the windscreen wiper Pressure Device motor- it increases the pressure on the driver's wiper arm
to ensure a perfect wipe at 10km or 200km/h. It winds up dependent on speed, if it gets water in it it corrodes and then makes funny noises that appear to be from the dash (but are actually from the plenum chamber under the windscreen).
Get to it early enough adn all it takes is a new gasket, clean out adn regrease, too late and the whole thing must be replaced (new from the stealer: so $$)

Lol, I threw a few of those ebox blowers away last week..."what the heck are we going to do with them?"

[wingman]

anyone got a pic of the ebox fan?