Sorry, but it sounds as if the AF is broken or worn out. The "clicking noises" do suggest a mechanical failure. But to be sure, if possible, try the lens on a different camera to see if it works properly. Also, carefully clean the lens-to-camera contacts on the rear of the lens. Just use a clean, lint free rag lightly dampened with a couple drops of isopropyl (rubbing) alcohol. Also operate the AF on/off switch a dozen times or so... those switches are usually "self cleaning". If for any reason they get some oxidization or dirt inside, operatiing them will clean them.
To be honest, I don't have much hope that any of the above will "fix" the lens, because the noises suggest it's trying to AF, but failing. But it's easy and doesn't cost anything to try any of the above.
If none of the above helps, you can look into repair, but unless it's still under warranty it might be more cost effective to simply replace with a new lens. Depending upon which version of 18-55mm yours is, a newer one or some other lens alternatives might be a nice upgrade anyway.
But first, a common reason the AF system gets damaged is manually overriding the auto focus of a micro motor lens without first turning the AF off at the switch. Canon warns not to do that with their micro motor lenses. On the other hand, their STM (stepper motor) and USM (ultrasonic) lenses both will tolerate manual override of AF without need to first turn it off. So, if yours is the micro motor version (i.e., it doesn't say STM on the lens... there is no USM version 18-55) and you weren't careful about turning off AF before focusing manually, this might have been the cause of the problem.
Currently there are two versions of that lens offered: EF-S 18-55 IS II sells for about $200 and EF-S IS STM costs about $50 more. You'll have to compare the cost of repairing the one you have, with these prices. But I also believe both current 18-55s are optically improved from the earlier version. In addition, the STM has improved AF performance (faster, smoother, quieter), a non-rotating front barrel (preferable if using filters such as polarizers), and tolerates manual focus override.
Or, if your budget allows, you maywant to consider a further upgrade such as the EF-S 18-135mm IS (also avail. with STM for about $50 more), EF-S 15-85mm IS USM, or EF-S 17-55/2.8 IS USM.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
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