Learning the difference between aircraft engines

During the Blitz and following raids, my grandmother would put my father (born 1937) and his younger brother (1938) in the Morrison shelter at the slightest sound of aircraft engines. As the war progressed, the boys learned to discern the lower note of outgoing Allied bomber engines from the higher-pitched incoming Germans. If they were Allied, they wouldn't worry.

However, this could be a memory transferred from 'Beyond the Fringe' or 'The Goon Show'. One of these radio shows had a radio skit where a character describes Lancaster engines as sounding like friendly old bumblebees whereas Heinkels sounded like nasty whiny mosquitos.