I have this HORRIBLE racket on rough roads, which I attributed to completely shot shocks and it being the wire wheel covers banging around because the wheel is obviously hopping too much.
HOWEVER, I have noticed that the sound is negated when on the brakes. A little bit of brakes doesn't stop the noise, but a good solid brake force will have the noise gone.
So this leads me to believe that either the pads, or the calipers themselves are just banging around. I have only had the wheels off...
Front calipers/pads, do they have anti-rattle clips?
