How to stop my toddler from head-banging in her crib?

My 2-year-old has been a head-banger for most of the last year. When half-awake she rocks on her knees and bangs her head against the side of her crib repeatedly. It can go on from anywhere for a few seconds to half an hour or more. This is annoying for two main reasons: 1) it is noisy and keeps us awake. 2) It makes her hair a rat’s nest and impossible to brush. You can only imagine the hair-brushing wars we have at our house. I have shed actual tears over this. Her pediatrician says that though annoying, it is nothing to worry about and she will grow out of it. I have tried waking her up and resettling her with a reprise of the bedtime routine. It seemed to work for a couple nights, but now she is teething and the banging is back worse than ever. Any experience with this? Ideas? Help!