When i turn my cameraon the lens opens ans then shuts automatically with an e18 message on the screen and then makes a strange whirring noise

It always comes because there is something loose inside the lens, such as a screw. These articles maybe helpful for you: Lens error, E18 error and turn the power off and on again Good luck

Lens errors (E18 errors on Canons) are fairly common. Usually it's sand or grit interfering with the lens extension mechanism. Or the camera's been dropped with the lens extended. Or the camera has been powered on, but the lens had been blocked preventing its extension. Or the battery ran down with the lens extended ...

Here's some things that you can do to try to correct it. They only seem to work for less than 50% of the lens errors, but if the camera is out of warranty, they're worth a try:

http://camerarepair.blogspot.com/2007/12/fixing-lens-error-on-digital-camera.html

The E18 error is an error message on Canon digital cameras. The E18 error occurs when anything prevents the zoom lens from properly extending or retracting.[1] The error has become notorious in the Canon user community as it can completely disable the camera, requiring expensive repairs.
A quick internet search for "E18" + "canon" quickly reveals hundreds of similar complaints.
ConsumerAffairs.com reports that the "lens has a feature called bellows claw, which is a gear that physically extends and retracts the lens. A piece that holds the lens, the barrier plate, is not large enough and can sometimes cause the bellows claw to malfunction, resulting in a stuck lens". The result is a black screen that only contains the error message, E18. Another problem mentioned on the site blames a sticky iris in the lens, caused by grease entering inside from the microphones built into the lens. The buildup freezes up the ability of the lens to open.

Canon E18 error happens when the lens gets stuck while trying to extend. The camera will beep a few times and the LCD will display a little E18 in the lower-left corner. The lens gets stuck in the extended position, and refuses to move either to focus the lens or to retract when powered off.

take a look at e18error.com website for more info

ah ok

http://blogs.yucs.org/~dwallach/2004/08/canon_e18_error_repair_instruc.html
The better solution is to send it to Canon's service people.  They can fix he problem is the fixes at the above site won't work.

this is very easy to fix- switch your camera on, grab then lens around the edges, and carefully give it a firm twist. It should click and turn on.

Sounds like the gears are stripped in your lens. Send it into repairs and they will send you an estimate on what it would cost to fix it. Give them a call at 800-706-2534 option 2.

The E18 error is fairly common. Here's some things that you can do to try to correct it. Have already fixed several cameras with this problem by using these methods:

http://www.e18error.com/repair.html

The "E18" error code indicates an error that involves the lens unit or lens cover. The grinding noise you hear-to my regret- means that len's mechanism is malfunction. This type of problem could be fixed by professional only.