Why does a car door make a 'popping' noise when opening or closing?

It's quite a common problem when you open or close the car's door and it makes a loud popping or clicking noise. There could be several reasons, but the most common culprit is the part called 'door check strap or arm' or simply 'door check'. It's a little arm that limits how far the car door can open. The door check also 'holds' the door in the open position.

What causes the door check strap to start making noises?
Door checkDoor check strap (arm).
The most common reason is when one of the bolts comes loose. The door check is typically held by three bolts/nuts. We have seen many cars, when the loose bolt number 1 (in the photo) causes a popping/clicking noise. So, the first step is to check if all the bolts 1, 2 and 3 are torqued to specs. If you want your mechanic to check the bolts, just mention it when you take your car for a regular service. Typically they won't charge you for tightening of the bolts.

The second common problem with door checks is when a popping noise is caused by a loose pivot pin. Typically it's visible when slowly opening and closing the door if the pin is loose. Often it might look rusted. In this case, the door check will have to be replaced.

The third common problem is when the door check strap becomes noisy due to looseness in the strap itself due to wear. In this case, the door check strap will also need to be replaced.

Other issues that can cause noises when opening or closing the doors include lack of lubrication or rust at one of the hinges or damage or cracks in the metal panel where the door check is attached.

Door Check Replacement: The part is not very expensive and in an average car will cost 0.5-1.0 hours of labor to replace. An aftermarket door check arm will cost from $20 to $35.
Door check armDoor check strap (arm).
Typically, the access to replace the door check strap is from the inside of the door, so the door cover and possibly some other parts will need to be removed. In many cars, the door check strap can be accessed through the door speaker opening after it's removed.
Do door checks need to be replaced on both sides at the same time? No it's not necessary.

Door check strap/arm maintenance: The only maintenance the door check strap needs is the lubrication of the pivot pin to keep it from rusting and wear. However, once the door check becomes noisy, lubrication will not help.