Peugeot 308 knocking sound

A bit vague!
Might help if you could isolate when it happens,
But anyway,
it kind of sounds like the anti roll bar "D" bushes.
Which can be difficult to asses so I normally remove the two clamps and let the roll bar drop down and then check the D bushes that way.
And as the D bushes clamp to the body it does tend to knock through the floor so difficult to pin source of knock down.
Not a big job to drop the bar and check and not expensive for bushes so I'd buy a set if I were you and change them anyway.
They're a common MOT fail point anyway.

Have you had a look at this similar problem:

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I have a 2012 1.6 308sw access. At around 16,000 miles it started to knock over bumps and ripples. noticeably at slower speeds, where you could hear it. It was checked out at a main Peugeot dealer after a mechanic took it for a drive with me. He was at first not sure what the problem could be
I was then told that Peugeot looked on the dealer faults database to find the solution. It was the front offside main engine mounting that needed repacking with grease. This resolved the problem. And from what I can gather is a known problem. On another note when they did the MOT at 22,000 miles they said I had a worn inner nearside steering rack ball joint. but not an mot failure. Since ownership at 15,000 miles I have had a slight vibration /wobble at speed and the steering is nowhere near as precise and stable as my very old 406 estate. Took it back to dealers several times and they fobbed me off and charged me for new front disks. which were worn when I first bought the car from them. they then said the problem was due to wheel imbalance which helped a bit but steering is still not as precise as I would expect. Maybe this is due to the slightly worn steering rack joint. Not that happy with the Peugeot dealer experience.