Yup, my guess would be the steering wheel's contact ring. It's what causes mine to do it every while. With my car, it only happens with a couple steering wheel positions, just left or right of dead center.

Does the wheel squeek when you turn?

__________________
1982 240D, sold 9/17/2008
1987 300D TurboW124.133 - 603.960, 722.317 - Smoke Silver Metallic / Medium Red (702/177), acquired 8/15/2009
262,715 and counting