Random knocking noise like something is banging on the bottom of the car

[Akos2210]

Hi!
I Have a C4 II 2018 and in the last few months I've been hearing a strange knocking noise coming from under the car though I can't pinpoint it well. It usually doesn't happen in the morning when I start the car, but it will come up after 10-15 min of driving. I couldn't tie it to any specific behaviour, though it seems to happen less when it's cold outside.
The knocks are random, though it does seem like they are connected to the turning of the wheels, and they don't happen when the car is stationary. They are independent of acceleration, and will also happen if I let the car just roll in neutral. They will also happen on completely level, good quality road. Two different mechanics told me they couldn't find any problems.
They are not very loud and passengers tend to not notice unless I tell them to listen, but it's annoying the hell out of me.

[GiveMeABreak]

You really need to get a mechanic to go out with you on a road test when this is happening to see if they can identify the source, because knocks can be from anything in the suspension geometry, suspension components or driveshafts and steering rack for example. So not something we can do on a forum to be honest - it needs a road test and checks on a ramp.

[PaulC5]

We had a knocking on a 2015 C4 and that turned out to be wear within the front left shock absorber so that the chrome shaft could move slightly sideways. It was difficult to check on the car but once I had the strut and shock absorber off the car the shaft could be moved easily and give the noise. Possibly with the car jacked up it may be possible to check for this without any dismantling. The Citroen parts department at our dealer said there had been a bad batch of shock absorbers and they had changed the design since. I replaced the part in 2019 when the car was on about 25k miles. The right side was ok but I also replaced that one so they were a matched pair.

[Akos2210]

Thanks a lot, this sounds like it could be it. I’ll get it checked

[Steve Walsh]

Also check for loose undertray and / or wheel arch liners

[Mabbs04]

Did you find out what the sound was? as I'm experiencing exactly the same and struggling to pin point the issue.....

[Akos2210]

Did you find out what the sound was? as I'm experiencing exactly the same and struggling to pin point the issue.....

Hi it turned out to be a problem with the steering column, for some reason the rack-bar was running dry and had to be relubricated.