It's tough when the noise is intermittent. If you could have someone
listen from outside the car when it is making the noise you might be
able to pinpoint the source. I've had a noise like you describe that
turned out to be the plastic trim surrounding the steering column
touching somewhere near the back of the steering wheel, but it was
pretty obvious that it was coming from inside. Just exerting a little
force on it made it stop. I also had a similar noise from under the
hood, which was caused by low power steering fluid. I used some with a
leak sealant, and it's been OK now for over a year.

David