My LF cover was rattling so bad people would look at me driving by at the mall.
I didn't want to risk breaking it, so I took the wheel off the truck and worked from the back side. You will be able to see how it's attached and carefully unmount the cover.
I chose to put a small dab of RTV at each mounting point after cleaning the wheel. If I need to remove the cover again, I can simply cut the RTV with a razor blade.
Six months now, rattle free!