Mine were still under warranty. It's hard to say if they always made the noise as I fitted them in the summer and the noise would only arise during the colder months then go again when it warmed up. I sent BC the rears back for them to inspect and bought another set of bare shocks (no adjusters,springs etc) from them under the proviso that if they found a fault with my originals they'd refund me (I think they were only about £70 each). In the end they could not find a fault with them but they gave me a refund anyway due to the fact that the noise totally went away after fitting the new ones which kinda proved there was a problem.
BC will ask you to do a few checks 1st, something like check top mount nut is tight,ARB links are ok (I disconnected mine to rule them out) Pre load is set correctly, Locking and adjusting collars are tight. I also tried them at different height and shock stiffness setting, the stiffness made a little difference I.E the softer the setting the less knocking they made but I found setting them below 10 clicks meant they were to weak to control the springs and lead to seriously dodgy handling. I did find that raising the car significantly reduced the noise but that kind of defeats the whole point of coilovers.
I seem to remember that I did a quick Google search on BC racing shocks being noisy and found alot of post on the subject from alot of different car forums.
My own opinion is that BC Racing coilovers are very good value for money and have an excellent back up service but when I next buy a set of coilovers I'll probably go for a top end of the market set.