"Heel-clicking" here is not a noun, but an adjective, along with "head-nodding". Heel-clicking and head bowing are well-known features of military salutes in the German military since Prussian days in the 19th century, and these have spilled over into civilian life. An Anglo-Saxon knows exactly what the speaker means which he refers to these, but to an Israeli who is less familiar, it's better to state "האמריקאים צחקו על הגרמני על גינוני ההצדעה התיאטרליים של הבוסים/מפקדים שלו.