2002 Volkswagen TDI

My 2002 Beetle TDI Desiel makes a knocking or fluttering noise at 60-70 miles per hour like a knock or thump. It sounds like its coming from the engine compartment down low. Souinds like someone sticking a stick in Fan blades. It has done this a number of times over the years and has just happened again. It stops if you slow down.

Any thoughts