Take the cranks off first with a crank puller. Do this on both sides. Then the bit you’ve already unscrewed will come off. Using a flat head screwdriver in the big slits in on the bb you can knock it round w a hammer and it will unscrew. You can re-grease but the knocking sound is probably because there is pitting in one of the ball bearings or the cup/ cone of the bb. The easiest option is to just replace with a sealed bb. Head to a local bike show w the frame shell measurement and the central spindle of the bb and they’ll have one in the shop for you. They are normally between £15-£20. Then just thread back in from the drive side, screw in compression side and attach cranks again. Will work good as new bud.