After a while your rubber inserts under the striker gets squished/looses it rebound to hold the latch tight and the 2 loops that are used as springs get deformed as well.
If you are tired of the rattle and want a cheap fix here is one, you can go get new ones but they will do the same as old ones in a few years, or you can do my mod and it'll last forever.
1st removed the 2 bolts holding the strikers...
2nd disassemble the parts
3rd here are the parts and how too do it.



Go get or find lying around pusher spring like this one, was longer but was cut shorter for mod, one with a flat side to go against the metal part above the rubber piece, cut side against the rubber.



Spring goes in between the 2 rubber loops sticking up.



This metal strap goes above the rubber part and the new spring.



It'll look like this.



Put all the pieces back together and put them back on either of the bed sides and adjusted it so tailgate when closed will be flush, after I did this tailgate barely moves.





Ps. Small pusher spring from pens will not work, need springs as wide as space between the rubber loops. Have fun!!!!