C5 II Strange clunk from gearbox

[dulenator]

Hi all!

Old car, new issues.
I have had this problem for almost a year and finally have decided to try and resolve it.

When I start the car and start moving, at exactly 20km/h there is some clunky noise, like something slipped into place, coming from the gearbox side. I can even feel a little shake on the pedals when that happens.
It does not repeat until I come to a full stop and turn off the engine.
If I don't turn off the engine, the sound does not appear again during the entire drive, whether I come to a full stop or not.

It doesn't matter if the car is in gear or in neutral, clunk is always there (once) at 20km/h.

I've been to just one mechanic, who is also my friend, and he can't say anything without disassembling the gearbox.
The car drives without any issues regarding the gearbox. The clutch is ok, and shifting is great, so we would like to skip any unnecessary gearbox work if possible.

I've done more than 20,000 km with that clunk and it does not get worse or better with time.

I'm sorry for the long post, but it's been bugging me for a long time, so it would be nice if someone said it is a known issue and do that and that and you'll be good.

[Rp0thejester]

I would personally check the handbrake cable to the rear brakes. It could be sticking on one side and make the clunk noise once it's released the brake. It would only happen at the start of a drive not during unless your stopping alot

[dulenator]

It's not related to the handbrake because we recently changed almost everything around it, and it is finally working as good as it can for a stupid front-wheeled handbrake.
It is not sticking, and it just passed an annual vehicle safety test.
The sound is the same as before the handbrake job, and it is coming more from the gearbox area.

[PaulC5]

Is the battery loose ? That is in the gearbox area if it is your 2005 C5 and it might take 20 km/h for it to move slightly. Maybe there is a problem with the starter motor.
The rear brakes on the C5 to 2008 models can clunk and there is a shim that Citroen do (440721) which clips over the pads but is not part of the brakes when the car is made. I fit this to our 2004 and 2007 C5s. It can be seen here as part 8 download/file.php?id=8893&mode=view

[dulenator]

The battery is firmly secured, and the sound is definitely coming from the front left side, where the gearbox sits, so it couldn't be the rear brakes. I hear those when they get stuck and it's definitely not that.
I'm planning to change the clutch assembly sometime next year, so if I don't find out what is making that noise, I'll probably find out then.

It is just interesting, at least to me , that this happens always at the exact speed and only once, until the car is stationary and the engine is turned off.
Going uphill, downhill, sideways or any other way doesn't affect anything.

If it was some engine/gearbox mount it would clunk all the time and would "reset" itself during braking or engine moving, but it does not.


It's hard to find a knowledgeable mechanic, who will fix the issue without going through the system of elimination, thus making the repair more expensive than it should be.
That unfortunately seems to be a universal problem all over the world, and that's why we are all trying to do as much as we can by ourselves.

[xantia_v6]

Could it be something that the suspension is triggering?

HA3 or HA3+ ?

[dulenator]

HA3
I think that there's nothing on the front left side besides the shock that could do something. All of the electromechanical suspension stuff is on the right.
If no one finds a solution, I will put out a quiz, when (and if) I find out what it was.

I don't have time (or cash) at the moment for the clutch job, so it will have to wait for another 6-8 months or until that clunky thing finally breaks.

[dulenator]

Just a little update on the issue - I think this may be related to the ESP/ABS thingy because today the same clunk happened when the wheels slipped and ESP was activated.
Also, I feel a slight twitch on the brake pedal, when I hear that sound.