remush

Hi all, I went to look at, and test drive a 1999 Toyota Hiace LWB Van not far from where I live. Didn't look to bad for its age, done 214000km. Its a Petrol Auto. Guys got some receipts for work he's had done to it. I noticed that when it changes from 1st to second gear there was a clunk sound coming from the back of the van. Any ideas what that might be ? All comments and suggestions welcome. Thanks in advance for taking the time to read this post :)