When I had a booming noise that seemed to blow back throught the air box on my 115 51 plate tdi with 132 k miles it turned out to be wear on the cam lobes. There was no smoke and it started just fine but just sounded like a tractor also. I had to have a new cam , cam shells, tappets, cam belt, water pump, air mass sensor as the back flow of air knacked the old one. Price inc a oil change at a VW specialist in Lincoln