Rattle behind dash, booster?

I have a rattle behind the dash on my S1, on the drivers side. It sounds as if the speedo had a bad bearing, but the noise is there whenever the engine is running and when I tap the gas pedal lightly the noise goes away. At idle it is not really there, but just off idle it becomes audible. It doesn’t change in relation to engine or road speed, and pushing the brake pedal has no effect. Once I open the door the exhaust is too loud to follow the noise to the source.

Since no tubes, wires or cables make the noise, could it be a bad valve inside the brake booster that isn’t sealing right and starts to chatter or vibrate when the vacuum is high? I don’t want to remove the vacuum hose for now since it’s not so easy to access and I don’t want to break the inlet on the booster. Anyone with a similar experience or who can point me to what causes the buzz, would be greatly appreciated. @peder , were you able to fix the Mk9 rattle?

Vacuum is the highest on idle.

You can remove it on the engine side, might be easier.

I had a very annoying similar rattle, it took me forever to find it, and it was the heater matrix hitting the box because the foam around it had completely deteriorated.

Check also the throttle and cruise control cables.

I read about your matrix but that wasn’t at all throttle related, was it?
I was thinking that the slight drop in vacuum may create a flutter in some valve (I don’t really know what a booster looks like inside, diaphragms, valves, filters is all the theory I understand). All the cables and even the brake lines have been ruled out, I stuffed foam in between everywhere forward of the bulkhead and that didn’t change.

The engine side has a clamp which is not easy to get at :frowning:

Throttle linkage inside the car might be another area to look at.

Thank you!

No, but mine is a V12 so no vibrations from the engine.
It was happening when on continuous little bumps, like on gravel roads.

I had a rattle behind the dash in the servo area on a LHD XJ but it was rough roads related. Turned out to a a small socket dropped crom a previous job…

Well, that can’t be it either. I conclude that brake booster valves are not a common cause for rattles.
I‘ll clamp the brake hose over the next days and see if that changes anything.

A few sockets would be nice, but I only found the last missing acorn nut in a crevice, covered in dust.

If your car is a manual transmission vehicle the throw out bearing is suspect. Touch the clutch pedal. Does the noise cease? Bingol irritating, but relatively harmless.


How would my long lost 3/8" get so far from home???


It’s an automatic. None of its noises fit in, it doesn’t change with temperature, load or gear - only goes away when I tap the pedal lightly.
I only lose 10mm sockets somehow… inches are all still there.

So you tap the pedal lightly and the noise goes away, does it come back immediately when the engine idles down?

I wonder if the ‘harmonics’ of the engine are just right to create the rattle at an exact idle speed?

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Yes, it goes away immediately but I also heard it while driving, so at a different rpm. Harmonics was my first thought too :confused:

I assume that the series 1 is similar to the series 2 in which case the top of the dashboard comes off quite simply and that would give you some better access for investigating (apologies if you have already done this!).


No, I haven’t done that yet but it seems like I‘ll have to tear into it next week. Been there on the S3 so it‘ll be fine. Only difference is the speaker and I have to look at it anyways.

Update: it’s clearly the booster. Further investigation when I feel like it. Everything functions, holds vacuum after shutdown etc. and I’ll read up first.

Solution: has to be the non return valve into the brake booster. It rattles when the vacuum is high and briefly goes away when the booster is evacuated. I can feel the rattle in the valve and it is not leaking anywhere. When the grommet has arrived I‘ll try to fix the old valve first.

And it happens to other cars as well. If you know what to look for there’s several people who had the same issue. What a relief that it’s a simple repair!