2001 Nissan Sunny power stearing problem

Your mechanic could be right, but I wouldn't agree with his diagnosis until I inspected the car myself. Rack and pinion steering gears rarely make noise without there being some other real obvious steering problem. Also, it is extremely rare for a mechanic to rebuild a steering gear. If the parts are even available, they will cost more than a professionally-rebuilt gear with a warranty. You'll pay more for the parts and labor than for the rebuilt gear, and you'll have to hope the work was done correctly and won't have to be done a second time. Leaking seals is the most common failure, but they usually start leaking because of nicks or corrosion on a metal part, or wear in the housing that allows a metal part to move away from the seal. Rebuild kits don't include those metal parts, so that damage will destroy the new seals. Those are the three things your mechanic is worried about. Parts cost more, he might make a mistake, and the new parts might get damaged right away which may not be his fault. All of those things are addressed by the professional rebuilder. Those rebuilt gears are real inexpensive today.

The more common causes of rattles and clunks are worn ball joints, tie rod ends, lower control arm bushings, and anti-sway bar links and bushings. I would have the vehicle inspected at a tire and alignment shop for a second opinion. They're experts at finding the causes of rattles and other noises.

Since you're only looking for the cause of a noise, here's something else you might consider. There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

The tool is available from the guys who drive the tool trucks to repair shops each week, but you can also find them on Amazon for about half the price. The original model used six wired microphones. On the newer model four of them are wireless.