Sounds like an out of round tire.

If you can put it on jack stands and rig up some kind of runout gauge you
can see for yourself.

Rotate the front tires to the back & see if it goes away or moves from the
steering wheel to your butt.  That'll tell you is its a tire/wheel or a
front suspension issue.

On 4/7/08 11:32 AM, in article
d96466a0-9cef-4e60-b59f-c4342990f133@e67g2000hsa.googlegroups.com,

antmorano:

[...]

There may be more than one cause for the vibrations, and the causes may
function simultaneously.

A faulty shock absorber could cause wheel hop vibrations.

Vibrations may be caused by a rippled road surface that had been
distorted by the periodic vibrations of vehicular traffic. The car's
wheels will bounce according to the waviness of the road surface if the
vertical wheel motions are not damped by shock absorbers. The dampers of
most cars would damp the oscillations, and the vibrations wouldn't be
visible. Bad dampers may permit the increase of amplitude of the
oscillations, and the vibrations would build to noticeable levels. That
cause for vibrations may be specific to some roads and not others.
Harmonic oscillations of the vehicle structures and suspensions are also
the cause, and they also result in the vibrations of the structures and
suspensions of the cars that follow on the same stretch of road.

Wheel mis-alignment, for example toe-out/in, might also contribute to
the problem. Especially on undulating road surfaces. If so, unequal tire
wear may be one result. Nissan could check that on the chassis machine.
Inspection and measurement may show suspension or frame damage. On the
other hand, a simple steering tie rod adjustment may solve the problem.

Ralph Hertle