There is probably something not bolted correctly and is coming loose
Worn Ball Joint
Depends on where the sound is coming from, and year. I have an 2000 and it ratles in low gears. the problem is the heat shield under the driver on the exsoust.
It is your heat shield making the rattling , you can go to lube xpress are anywhere with a lift. They can take that off easily for you. Another rattling problem: My 2000 Ls sounded as if the front end was falling off when I hit small potholes or while driving on rough surfaces. I found that the front anti sway bar bushing was worn allowing movement. It wasn't noticeable by hand. The bushing are not available for replacement they come as part of the sway bar itself. I purchased one at a local Ford dealer for $125.00 It took 6 hours for me to replace it and had to pull it out from the drive side wheel well. It was very cumbersome to do but it eliminated the rattling completely.
mufflers have fan pins on the inside, one could have broken off and once you get to a certain speed it would rattle
you now get over it
Struts
Heater Core is the likely culprit
The bearings might be bad in the alternator.
My 2000 Grand Prix would have a low-speed knocking noise coming from the drivers side as the CV axle was failing. The loss of lubrication in the CV Boots allows the joint to make a knocking/tapping sound at a low speed. If the knocking is coming from the engine compartment, then it could be an entirely different problem.
collapsed lifter?
While a rattling comming from the passenger side of any vehicle could be almost any mechanical failure it could be something very simple. I know that in most 2000 Mit. Montero Sports the Jack and tire changing equipment is fastend to the floore under the back passenger seat. Sometimes in can come loose after a few bumps in the road thus making a rattling sound that would come from the passenger side. You will have to fold your seat forward (the bottom part that you sit on not the back of the seat that you lean against) and then you will see the tire changing equipment. Just check to make sure all is secured. If this is not the the problem at least you have ruled out another possibility. Hope it helps,Amanda Shreveport, LA
Up front, behind the grill, but in front of the radiator. Stand in front of the vehicle and have someone else operate the horn - you will quickly locate the source of the sound.