I have the same effect if I put into nuetral. As I said before I had them look at the tranny with nothing found or disclosed. My mechanic inferred it was the band applied solonoid which I guess asssts downshifting in the tranny? I am suggesting the driveshaft yoke, or torgue converter. The clunk is more then noticable and has over time become more pronounced. Good luck with yours. I am taking mine in following the new year to get worked on. I will let you know what is figured or better, resolved.