My 1972 Citroen DSpecial 21 generates quite a lot of rhythmic knocking and rumbling, particularly on slowing down and braking, and slow turning/manoeuvring. The problem is presumably from the drivetrain - the knocking/rumbling is proportional to driving speed, i.e. slows as the car slows down.
I put the car up on axle stands and ran it in gear and had a look at things from underneath. I was looking to see if the discs appeared to have any distortion/run-out (which might have been where the problem was coming from as it is most apparent on braking) but they looked fine. However, there was quite a horrible kicking of the discs, diff, gearbox, etc, a few mm side to side, that seemed to be much worse from the LHS than the RHS (I held each drivewheel still to make the diff drive the other side, to see which side caused most kicking).
The LHS tri-ax housing turned out to be extremely worn, so I have replaced that with a very good condition (no wear in the ball runs) second hand replacement. However, there is still some knocking and rumbling, and I am presuming the tri-ax on the other side is similarly worn. So, I intend to check, and will probably need to replace, this one too.
However, I wonder if anyone can tell me if there might there be something else causing this kocking and rumbling, aside from the worn (probably on both sides) tri-ax housings? I did notice that the differential seemed to have quite a lot of "slack" in it (when turning each of the driveshafts by hand) but I'm not sure what is "normal" with this, and whether this would lead to knocking and rumbling through the drivetrain anyway, unless it is so worn it is about to fall to bits?! Wheel bearings feel smooth, and steering ball joints, etc, all feel fine (one steering arm ball joint was worn, which I have replaced).
Any comments/advice would be much appreciated.