Using two cell phones with gStrings app to use tuning fork sine wave tones to input to digital oscilloscope, center both waves on centerline, then select XY mode - I noticed the slow waveform pattern movement. Oscilloscope locks trigger on one waveform and if waves are not phase together and slightly different frequency one sees pattern flatten out then go through looked for loop pattern.
Consider watching Lissajous waveform on scope for a time period with less than perfect circumstances.
