I have aquired a 2005 GTX NA with 50 hours that had a bad temp sensor and display. Got all that worked out. The problem is there is a loud rythmatic thumping noice I hear and feel in my seat at idle. Its loud. Like a lose motor mount bumping up and down. Above 2000 rmps it goes away. Changed plugs, pulled pump. Oil level is good. Checked motor mounts, all solid. All looks ok. Out of water it is quiet. It runs hard. Idle is a bit rough. Very puzzlilng. Was wondering if maybe it was knock sensor or maybe just a missing causing it. Any ideas? Thanks guys.