I had nearly the exact same sound on my 2013 FX. Harlan at Hollywood Electrics suggested it might be a loose front pulley, and he was spot on! I loosened the belt tension, slipped the belt off the rear pulley, and could feel the play in the drive pulley with my hand.
To fix it, I eased the belt tension, slipped the belt off the rear and front pulleys, removed the mounting bolts for the motor, removed the drive pulley setscrew and mounting bolt (yes, it was loose), and removed the pulley and its key. I applied a liberal amount of Loctite red (no, I never plan to disassemble this) to the motor shaft and keyway, reassembled it, and applied some Loctite red to the pulley bolt threads and setscrew threads, and tightened it all according to spec. Re-bolted the motor in, put the drive belt around the pulleys, re-tensioned the belt, and test-wheelied the bike. All good!
My sincere compliments to Harlan at Hollywood Electrics for steering me right. I've bought nearly nothing from him (yet!), and I live far away in Colorado, yet he still took the time to give me excellent advice, enabling me to get my baby running quiet and smooth once again.
Hope this helps,
- Fred