What could this knocking noise be ?

[Tony Russell]

Hello All,
What kind of electrical component sits (probably) between the bulkhead and the console, in the centre (ish), is powered up when the ignition is switched on (ie dash lights up, before the starter is turned), and can produce a knocking at around 1Hz for about a minute ?

Frequency is not related to engine speed once the engine is started,
It's not the windscreen wipers,
It's not the heater fan,
There's no air conditioning,
It's not music from the radio.

I was thinking some charger for the power steering, brake servo or something.

It's a diesel A Class thing BTW and it jjust appeared this morning.
I have stopped and started a couple of times, and each time it is there.

Bound to be something very simple, and a lot cheaper to sort now than wait a bit for it to do someting daft.
Can't, for the life of me, figure out what it might possibly be.

 

[moonloops]

1hz you say ? Now if you had said 1.5hz I could have told you exactly what it was

More seriously - is it the heater flap control rods - a regular knock sound that stops after about a minute. Happens on several other mercs...

 

[Tony Russell]

Tell me more.
0.8Hz might be more accurate BTW.

 

[moonloops]

http://www.mbclub.co.uk/forums/interior/77043-knocking-behind-dash-anyone-help.html

??

 

[Tony Russell]

Ah.
I checked my wifes car, same type, and sure enough, something engages quietly at ignition on.
Then I checked mine, adjusting the flaps as it clicked - and there was a change to the tone if I was moving the flaps as it knocked.

So we are in the right area - thanks for that !

So I guess, bearing in mind I have learned the hard way not to try and save cash by saving cash, the best thing to do is to take it to my local indy guru and have him diag it for £40, then take a decision on whether to fix it myself, give him the money to fix it, or live with it.

Funny thing is, in retrospect, I could swear it ties up with me changing a headlamp yesterday.

I read elsewhere (Bert Rowe's site) that sometimes you need to reset the ECU when you do some electrical things.
Something to do with switching on, turning the wheels left and right, then opening and closing the windows.
I wonder if there is the slightest chance that this might be related.

I'll have a look at that and let you know (worth a try).

 

[tpwuk]

I may be way off the mark here but is it someone at your front door? Jehova's Witnesses maybe?