When i first had my Z4 there was a banging noise from the rear right side, sounded like someone hitting the underside with a rubber mallet.
Took me nine months to cure and two unsuccessful visits to BMW dealers (So much for warranties), after i replaced the anti roll bar bushes for poly and replaced worn out standard shocks with Bilsteins.
I finally cured the problem by rotating the spring so that the number and letter printed on the spring went from outboard to the front.
Never had a noise since.
Apparently by chance i came across a BMW technical advisory from some earlier BMW models that had a problem with knocking. It suggested applying talcum powder to the rubbers at each end of spring, which it sits on. Which i was i did.
I removed the spring, washed the rubbers, applied talc then reassembled, but by mistake positioned the spring 90 anticlockwise when refitted. All this cured the noise. The tech bulletin suggested the barrel springs were gripping the rubbers and rebounding.
I think the barrel spring is a poor design. They compress in an arc. Unlike the front springs. It takes too much load on one side over a small area, hence earlier models breaking and as the compress the ends tighten and grip the rubbers causing a rebound.
