Looks like mentioned, it has rubbed somewhere at some point. It could have put a small slice in it then cracked and got bigger over time.

It doesn't take much weight missing to cause a vibration, like it a 1/4 ounce wheel weight fell off... so it very well could be why your noticing it now.

68 Road Runner, 69 Belvedere, 71 Challenger Vert
340 barracuda, 01 Ram CTD, 95 Ram, 04 Ram, 85 Daytona turbo Z
66 GTO, 06 Magnum RT AWD. 07 Ram CTD, 07 Ram