I hear a clunk in rear of my 2000 Nissan maxima
Noises like that can be a pain in the rear to fine. It could be something as simple as an item lying loose in your trunk or something more serious underneath the car like a suspension component.
Try bouncing your car up and down by hand and see if you can hear the noise and determine where it is coming from. Inside or outside or underneath the car. That would help in knowing where to start looking.
If not the first thing to do is check your jack and spare tire and make sure they are well secured and not the cause of the noise. Make sure the springs for the trunk lid are secure and the trunk lid hinges are tightly bolted to the lid.Then check all around, especially down in the fenderwell areas and behind the seat for loose items. You should also check for loose light bulb assemblies, gas feller neck, and even the lock assembly on the trunk. Oh, and check the rear speakers and package shelf.
If you find nothing inside the trunk then we'll assume the noise is coming from underneath the car. You'll need to raise the rear of the car up high enough to crawl underneath and then place jack stands to secure it. We don't want it falling on you. Also remove the rear wheels so you'll be able to easily see the suspension components.
But before removing the wheels, place your hands in the 6 o'clock position and wiggle the tire in and out. We are looking for any play in excess of say 1/16" to 1/8" or so. Anything more could indicate worn or out of adjustment rear wheel bearings which could cause our noise.
Look at the mounting points for the struts/shocks, the sway bar, and any other suspension components. If you see red rust that generally would indicate worn components and possible problems. Look at the rubber bushings at the mounting points, those are the most likely places for wear and noise. If they are cracked or missing that's your most likely culprit. And if you need to change one, just change them all because they are all the same age and will be in the same condition as the bad one soon enough. Check the struts/shocks for leaking oil. If they are leaking they won't stop the car from bouncing and can let it bottom out. Look at the rubber bumpers mounted most likely on the frame that are there just for that purpose. If they show signs of damage that is an indication of poor performing shocks, bad springs, or maybe just overloading the car. But it needs to be checked. Grab the struts/shocks, the sway bar and other suspension components near their mounting points and see if they move. Just like the wheels, a little play is allowable, but not a lot. If you need to use a long screwdriver of pry bar. Be careful not to dent the struts/shocks tubes.
Also check the exhaust system for signs that it has been hitting the frame, rear axle, bottom of the car, or some other components. There could be broken or missing exhaust hangers allowing the exhaust pipes to float around and cause the noise.
What you are looking for is something loose, broken, worn, or unattached that is able to make the noise you are hearing. If you don't find anything then your best bet is to go to a local garage that does suspension and exhaust work. If they sell tires that helps because they will be used to working on these kinds of problems and have the equipment to handle them. Good Luck!