Sounds like the Splutch nut may be coming loose. Watch the video below and check it first.
If that Splutch nut is loose and coming off, I'd suggest going to your local hardware store and buy and apply loctite blue 242 threadlocker and re-tighten that nut to keep it from loosening up again.
Here it is: Threadlockers from Loctite Products | Threadlocking / Mechanical Gasketing | Loctite Adhesives
model is whirlpool wtw5640x0 serial is c04031578. problem is nut wont budge but will not tighten either. when you turn the nut the spline for basket spins or something in transmission is free spinning. no way to hold the spline without damaging threads.