Any clues?
I am not familiar with the set up on a 406 but knocks can be caused by the strangest things. As an example, I had an Austin 1800 which had a very nasty knock from the rear suspension over the normal bumps you get in the road. It was eventually tracked down to the levers that projected through the brake backplates onto the handbrake cable. Their mounting pins had worn. The cure was to glue a piece of leather from an old belt onto either side of each lever where it went through the slot. The repair lasted until the car was scrapped five years later. The moral is that even silly little things can sound like something more serious so check everything like exhaust mountings, mud flaps and anything else underneath.
It can be usually 1 of 2 things. The most common is the rear track control arm. This is the arm that is adjusted for the rear toe-in, one either side. If you get hold of the wheel at 10 o'clock and 4 o'clock (on the N/S) or 2 o'clock and 8 o'clock (on the O/S) and push/pull there should be no play. If there is then they need renewing. The other thing to check is the anti-roll bar drop link. Not easy to check, but the wheels need to be on the ground and the drop link bushes (top and bottom of the link) checked for wear. Really best done on a ramp. I'm not saying that there isn't something else, like exhaust, or whatever, but those two are the most common in my experience. Good luck!
I had a similar thing and my regular non franchised garage gave up. (They hate Peugeots!) I took it to a main dealer and they diagnosed it straight away - replaced both rear anti roll bar links and one front link. It's now fine and no knocking whatsoever. If I remember it seemed worse going round uneven corners. Hope this helps.
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