I have heard several people talk about 'noisy cams' and lifters. What actually happens when cams make 'noise'? What causes cams or lifters to make noise at certain RPM's? Case in point, I recently installed Andrews 21's in my 88, and it is generally quiet until about 2200 to 2500 RPM, then rattle city. After that RPM spread, it gets quiet again. Why would a cam or lifter do this but not at other RPM's?