You didn't mention what lens you are using, and that might be relevant. For example, if your lens is an "IS" model, the humming noise you hear might be that working, not the lens trying to focus.
However...
1. In addition to checking that the switch on the lens is set to "On", operate that switch off and back on a dozen or two dozen times. It's possible for switches to get oxidization inside, interrupting the power supply. But most are self-cleaning when operated. Often time we just leave these switches set to the one position, so they never get "cleaned". Simply operating them a few times sometimes does wonders.
2. Remove the lens from the camera and clean the gold contacts on rear. To do so, I recommend using a clean, lint-free rag lightly dampened with a few drops of isopropyl (rubbing) alcohol. Be careful to keep off the lens optics while doing this. Also check the corresponding pins just inside the front of the camera. Those are spring loaded, and should all extend about the same amount when the lens is off. You can gently press on them individually and watch if they spring back nicely. Be very careful not to touch the mirror, shutter or anything eles inside the camera.
3. I agree, it doesn't hurt to reset your camera to the defaults too. In fact, you might even want to try to do a full "reboot" just to be sure some incorrect instructions aren't stuck in the memory (this is much like rebooting a computer to clear the RAM and caches... after all, a modern DSLR is a computer of sorts).
I am not sure this will work with the XSi.450D model because it has a built-in (non-user-accessible) secondary backup or time/date battery. But it doesn't hurt to try. To reboot the camera, first turn the camera off, then remove the LP-E5 rechargeable battery pack. Turn the camera back on, press the shutter release button once. The shutter won't fire, but this should drain any remaining power from the circuits. Turn the camera off, reinstall the LP0-E5, turn the camera back on and check the date/time. If it needs to be reset, then you got a proper "reboot". If not, well it was worth a try!
You also might try reinstalling the camera's firmware. You can download the most recent firmware from the Canon website support page for your camera, and very carefully follow the instructions you'll find there. It isn't hard to do, but must be done with a fully charged battery and careful attention to the instrucitons. If the camera shuts down or you accidentaly turn it off or otherwise cause the firmware to stop in the middle of an installation, the camera can become "bricked" and need to be sent in for service.
Neither rebooting nor reinstalling firmware is a guaranteed fix. In fact I doubt they'll help with most focus issues. But, what the heck, they are free and easy to try yourself... and sometimes solve problems.
4. If none of the above helps, try your lens on a different camera.... If it auto focuses normally, then there is most likely a fault with your camera and you will need to send it in for service. Also try a different lens on the camera, if it auto focuses normally, then the fault is more likely with your lens, which also would need to be serviced.
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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
FLICKR & PRINTROOM
