Sounds as though the washer is loose or perished at the base of the siphon and is passing water through in to the pan. It is not such an easy job but if you are mechanically minded you can try taking the cistern off the toilet (via the wingnuts) and screws into the wall and either replacing or remaking the sihpon washer in with silicone or similar. (be careful not to crack the cistern when tightening up again)
Good luck
Pete the Plumber, December 2010