I did the same thing, only it was when I added the lower lock kit to my XCT. That key rattled with the other key and fob on the key ring. I solved it by getting a split ring so while the key is in the ignition it is naked and I keep the other half with the other keys and fob in the trunk. When I stop I stick my helmet in the trunk, join the two rings and stick them in my pocket. It worked for me and solved the noise. For about 100 miles I thought I had done something bad to the bike. My last bike had a cam tensioner fail and it sounded the exact same way.
2012 Victory Cross Country Tour, Sunset Red.
Parts added to make her louder, faster and more mine.