When I am driving or taking off i hear a fast clicking noise as it is changing to its next gear but then goes away once it has change gears, mainly between 1st and 2nd gear and 2nd and 3rd gear, note it goes away once it gets into its next gear until its about to switch now when i'm out on the highway cruising at a steady speed 55mph to 75mph the clicking is constant. Any Ideas what this maybe cause the mechanic said is could be my exhaust manifold or something in my catalytic converter which i know is totally inaccurate cause i know i hear the clicking noise at about 2k rpms when it is changing to the next gear