This is a late reply however a valid one.
Today i visited VM Transmission Products to get my auto box serviced, flushed and an oil change. They came recommended by ADI. This issue has been discussed a few times before and recently in this thread Here
Basically the tapping noise has now gone and the Auto box feels much smoother. They charged £65 + vat and i supplied the Toyota Type IV ATF.
Thanks to ADI for recommending them and to Terminator for the original diagnosis (You were right all along), I wish i had done this sooner.
Thanks and Regards
ChrisR
P.S. I love this site, more than is healthy.