Not an easter egg as such, but I came across a BASIC program for the TRS-80 in the early 80's where the programmer had worked out that program lines of various lengths would produce interference of different frequencies when a radio was placed next to it.

Using lines which included long REM statements, and various loops, it would play the William Tell Overture.

For background, the Model 1 TRS-80 had virtually no RF shielding and would interfere with many audio devices next to it, so a radio tuned "off station" was ideal.