Sometimes It Makes This Noise

Your car makes a strange noise. You take it into the mechanic but the noise has stopped. You say "Sometimes it makes this noise."

The mechanic replies "Sometimes I can fix that!"

Once upon a time, many years ago, radios were different. A mother was at home with her young son and the radio went bang. She told her husband when he came home and he said Radios cannot do that. This was repeated several times. Then one day it went bang when he was home. Then he fixed it. I am the youngster, no longer 2 years old. Actually, 59 now. -- JohnFletcher

So what caused it?

It shouldn't work at all if a tube blew.

After some thought I finally had the notion that a small, infrequent spark might get amplified into a bang, since both are approximately impulses. Was that it?