Removing car body causes engine sounds to disappear

If you press Ctrl+W and change the "body" option to "empty" the car will not make engine sounds. You can clearly see that the engine is there, and the car will move and work like expected but it won't make engine sounds. If you change the option back to whatever it was originally, the sounds will come back.

 

I've noticed this since the first time I tried it in the demo. It's a shame it happens because taking a body off is a lot of fun.