See that AC line coming up from the bottom center of the photo? I added that black insulation on it (still has the sticker on the left side of the insulation). I covered the AC line and made sure it was thick enough to touch the bottom of the coolant reservoir to keep the reservoir from moving. I have done more driving and only hear the rattle when I hit a severe bump/pothole. It is 85-90% quieter now since I have added that insulation.
I circled it:
Some people have folded a sponge and stuffed it between that AC line and the bottom of the reservoir. I suggest doing this first as a test to see if it stops your rattling.
Mods: Drop in K&N filter, Steeda RSB, MS3 RMM, Remote start, Llumar tint, Ti engine cover, Husky floor mats, OEM splash guards