Air compressor makes loud knocking nois when running

SOURCE: Craftsman Air Comp. 6 hp oil-less, 33 gal horz. tank - sounds really loud

Sources of noise from craftsman oil free compressors (could be Made by Campbell Hausfeld or Porter Cable) include loose fan, failing bearing/s on the piston rod and housing, failing seal ring on piston, air filter intake and loose motor mounts. With the housing cover off, check the fan blade for cracks or loose screw. Turn motor by hand till the piston is at top of stroke. Examine the cylinder for scratches close to top. If scratched, air is leaking past vinyl ring making noise and causing longer fill time. If everything looks ok, suspect noise coming from bearings. Hard to tell if bearings are failing by looking at them. I have seen many failures of the piston bearing on the crank (fairly easy to change, I use alternator bearing from auto parts ) , however, the bearing in the housing just behind the crank bearing is much harder to replacable. Good luck with your fix-it project.