17-40 AF not grabbing focus

A couple of days ago I noticed that my 17-40 just wasn't grabbing focus properly. When in one-shot mode with the AF turned on and using the centre point, I try to get focus but find the lens goes in and out, sometimes settling on a spot and indicating (with the non-blinking dot) that it found focus--it doesn't, though. The AF sounds different from how it normally does--instead of just shifting to a position, it 'sawtooths' its way there.

I have tried cleaning the contacts with an eraser.

I have had a look at the rear element of the lens (or I should say the glass you can see when looking through the rear. There's no obstruction and it looks normal.

Any thoughts ?

is it new/under warranty? if so, send it in.

Nope--it's several years out of warranty.

Sounds like failing USM motor. If I had to guess, the repair may cost $150-200.

Hopefully not though and is just a simple repair. Sorry to hear