E87 120d - hissing sound from dashboard

[MitchT]

A week ago my 120d (E87, 103k miles, M47 engine) started making a hissing sound, not unlike the noise you get if you start letting air out of a tyre valve, but not as loud. It comes form the dashboard area. I initially thought it was air coming through the vents but, after closing them all and then turning the system off altogether, it continued. It starts when I start the engine and continues until I turn it off. When I turn the engine off the hissing doesn't stop dead, it fades away over a couple of seconds. The sound is constant regardless of revs, speed, gear, etc. Any ideas?

[rottenegg]

Does the hissing noise stop when pressing the brake pedal?

[LastPoster]

Does it have AC. If it does than shut it off completely

[MitchT]

rottenegg said:

Does the hissing noise stop when pressing the brake pedal?

No. It’s there from engine on to engine off.

[MitchT]

LastPoster said:

Does it have AC. If it does than shut it off completely

Yes to AC. On/off, makes no difference.

[rottenegg]

MitchT said:

rottenegg said:

Does the hissing noise stop when pressing the brake pedal?

No. It s there from engine on to engine off.

You said it "fades away" after engine off, so it's not quite binary is it. I've seen leaking brake servos do exactly that.

[E-bmw]

As it is a diseasel, it could be servo/vac pump or pipework in between, that would be my best guess anyway.

Can you hear it in the engine bay?

[MitchT]

rottenegg said:

You said it "fades away" after engine off, so it's not quite binary is it. I've seen leaking brake servos do exactly that.

Well, no, but it is constant from start to the point of engine off. It doesnt get stronger or weaker or change at all with engine revs, braking or any other factor.

[MitchT]

E-bmw said:

Can you hear it in the engine bay?

I need to check that, possibly after work tomorrow.

It's going to the garage next week anyway as the creaking noise during low speed steering inputs (separate thread) needs investigating and I've tracked that down to a possible top mount bearing... and the glow plugs probably need changing too.