Ajay, I do not have the slightest clue as to what the sound is. I started the bus, drove it a few miles, shut it down and it made the sound. I did the same thing again and the sound wasnt there. I'm going to spend some more time with this one, I am clueless as to what the sound is.
UPDATE: I took a further look and it looks to me like one of your engine mounts has broken off. I'm going to pull the piece completely out and place a new one in there. It'll take a couple of hours because I have to lift the engine slightly to place the mount. Mike and I will certainly spend a lot of time getting this one going.
Use 18501 until further notice.