Andrew,

you may be in the beginning stages of a very common failure of the top
steering column bushing. It's a plastic piece which eventually disintegrates
causing excessive radial movement of the wheel. It's easily and
inexpensively remedied by removal of the wheel and insertion of a new metal
bushing. You don't even have to remove the old, broken  bushing - it gets
driven down the shaft and out of harms way. Here's a link to a picture of
the replacement part:
http://www.pelicanparts.com/catalog/images/nori_images5/Bushing_92834773902.JPG

and here's a link to where you can purchase the part :

     Steering Wheel Repair Sleeve, 911/912E/911 Turbo (1974-89)
    F-347-739-02 $13.50

http://www.pelicanparts.com/catalog/shopcart/911M/POR_911M_miscel_pg9.htm#item40

This, of course, assumes that what you are experiencing is, indeed, the
bushing. I suppose other things could cause similar noise. If the steering
wheel does not appear to be loose at the top of the column, I'd first try
spraying some WD40, or other lubricant of choice, into the area through the
stalk openings & see if the noise subsides or changes.

Just did the job, myself. Easy & rewarding remedy to an annoying problem.