It's worth checking all the rubber bushes on the front end. I've just fitted Poly bushes to my E39. I used to be able to move the front wheels about 1" with a sharp kick to the tyre, just because of play in the front Tension Arm Bush.
If you get any play at all in either the wheel or any of the wishbones, I'd say it may be worth replacing the bushes.
Just one other thing. You said you've just put 18" wheels on. They aren't touching the wheel arch liners anywhere are they?