I've been asked: "What is that makes that distinctive VW engine sound?"

Some people (NOT me!) are keen to the different engine sounds of a Chevy, Ford or Mopar coming down the street. Some will even tell you if it's a BIG block or a small block.

Most anybody can tell the difference between a Harley and a Honda.

...and I'll guess that everybody can recognize that distinctive sound of a vintage VW.

What is that makes that unmistakeable sound?