Same thing on my '18 Deluxe. My dealer took my cap over to a press and tightened the crimp of the cap assembly. They felt fixing the problem was better than giving me a new cap that may do the same thing.

I had to get my password back just to thank you :D

I had to crimp the inner cap or whatever in a bit to reduce the rattle on mine but it's still there. But I think I have another rattle somewhere else in the front. I haven't narrowed it down yet. I'll have to get another cap to figure it out, which I was going to replace anyways. Mine pretty much rattled when I drove it off the lot I believe.

Mine rattled on my 2018 Fatboy. For the longest time I couldn't figure it out.
Held the cap and it stopped, however I was able to hear an even fainter rattle from the headlight surround.

2018 Sport Glide, had the same problem. For the longest time I thought it was something in the top end and felt like an idiot when I realized what it was. The dealer said he'd replaced three in the last month and replaced mine, but said Harley would only replace it once. The new one is already rattling, so I'll try some of the fixes here. Thanks to everyone who has shared ideas - you've probably saved me $50! (Incidentally, it's surprisingly hard to find gas caps with dummy cap mates...)

I changed mine with a cheap flushed one, no rattles here.

Ever since reading this I can't unhear the buzz. I keep forgetting to try these fixes....until I ride and hear it.

I used a screwdriver to tap around the perimeter of the underside after reading this last night and it got rid of the worst of it (though this second one wasn't that bad yet - the first one was really loud). Before I got the first one replaced, I wrapped a thick piece of nylon cord under the lip of the cap when it was in place on the tank and that did the trick too, though it wasn't the most elegant solution. But yeah, once you hear it, you can't ignore it. I can't believe this silly, simple problem can't be fixed at the source.

Where did you get yours? I bought one off eBay but didn't use it because it will funnel water into the gas tank if it gets rained on or washed.