Rattle noise through rear window

I believe I saw a couple other people complaining about a rattle noise coming through the rear window on bumps or whatever.
Think I figured it out!
Seen some people suspecting the tail gate and I never kept looking till today when giving a truck a quick clean.

I noticed the two plastic plugs in the corners of the back of the bed made allot of noise when I would slap the back panel. Temp fix in pic, put a piece of tape over the top..
I still need to test it, I would have waited but didnt have a chance.