i had a really annoying rattle in my boot. couldnt find it for ages. i found it was the little rubber stops on the hatch needed adjusting out a little bit as the boot was rattling when shut .the ones that sit on the rubber plates just above the rear lights when the boot shuts. also my boot catch was a little loose so just tightend it up. that seem to sort mine out. my golfs got rattles coming from everywhere. mined you i do live on a really bumper road. my transit van got less rattles than my golf.
Could be the tailgate rattling on the striker. When the rubber seals bed in, causes this issue. Like pd9264 says, wind the plastic stops out might solve it