I know you said new tire already, but my tank slapping always began when the tire was worn. So, next I'd try to adjust it out. Assuming the forks are healthy, personally I'd try to increase the preload so the rake gets a bit lazier. If you can get the back of the bike down, or the front a bit higher I think things would go away. The steeper the rake, the more propensity to be upset into slapping. Seem reasonable?

RZ Rob