FLSTFI Dave
I bought my 89 FLTC (Tour Glide) in 1991 with 4400 miles on it. It had (Tank Slap) then & had it checked out. Neck bearings adjusted & dealer talked to the MOCO & I was told, back in 1991 that motorcycles are not to be rode with your hands off the handle bars!
80,000+ miles, numerous front tires,tire pressure OK, wheel bearings, chrome lower legs. 23 years later, it still has Tank Slap on decel around 40 mph. It doesn't happen often but I'm aware of it!
As the MOCO stated years ago. Keep your hands on the handle bars! I have just learned to live with it!