Our geese will knock their beaks against our ford, tapping off a rythm that is actually quite groovy.  They also like to bite on the tailpipe and the fenders to make a grinding sound.  Harmonizing like this occupies our geese for quite some time.  They used to like to tap on the roof of the chicken coop (especially when chickens were inside - then even better noises are made!), and on the wheel barrow, and on any other hard, noisy surface.  But the truck, they've discovered, is best.  One or two of them will honk out rythmically as the others tap and grind.  We're going to try to get an audio recording of it...