Technical engine question on MGTD 1952. Original 1250cc engine, replaced cam, lifters, push rods. Had head trued up, new valve guides and seats. About 5 hours of engine time I hear a hissing sound from the engine. Can not locate. Wanted to eliminate possibility of loose head bolt, so removed the rockers to inspect bolts. While having all this off, I decided to inspect the tappets on the new cam setup. All are very nice showing good spin except for one. It is beginning to show signs of no spin. Pattern of cam on the lifter. Any ideas why one wound not be spinning. Thanks a bunch.