Yanmar Hammer Sound Normal?
The Yanmar diesel engines, especially the 2 cylinder models are fairly rough running engines. They have even inherited the nickname "Yammer Hammer" because of the distinct knocking sound the engine makes at idle. This noise if completely normal and, with decent maintenance, the tractor will continue to make that noise for 6,000 to 10,000 hours before a rebuild is necessary.
Below are 2 videos that I found online that show what the engines should sound like.
http://www.ymowners.com/YM1500-Clatter.wmv
http://www.ymowners.com/YM1500-StartUp.wmv
I do not know where they came from so I can't give credit to the original producer of the videos.