The buffer rattle is normal for the AR platform and is nothing to be concerned about. It occurs across brands and configurations.
Other things you will notice is a slight scraping sound as the weapon is cycled. This is caused by the spring scraping against the inside of the buffer tube as it is compressed and released, and this is also normal for the AR platform.
On occasion you may hear a slight "twang" during firing caused by the rapid compression and release of the buffer spring. Again, this is normal.
You can get third party buffer assemblies that eliminate these sounds, if you wish, but it would just be an added expense for no real positive gain.