my 2002 chevy malibu makes a loud pop but only when i reverse

There are a few possibilities, so I am going to include a walk through diagnosis link with this. When you go through it pay special attention to axle and suspension related sections. As the noise happens during back-up, an axle or suspension component, expecially a bushing, might be worn to the metal in the direction the it get pushed or torqued when it goes into reverse. While the forward motion or other contact points may not be as damaged or simply are not making the smae violent contact in that direction to make the noise.
The other possibility is there is a problem with the reverese gear in the transmission.
For these reasons, getting the trouble codes run is going to be very important as tracking down and eliminating all the possibilities will take much longer without doing this. Any Advance or Auto Zone, or large chain, parts store will retrieve the codes for free.
Get them and come back to us.

Here is the link for the walk through which also has a section on retrieving trouble codes.

https://www.2carpros.com/articles/popping-noise