[RFI] Noise - where do I start?

[Gary Peterson]

I have also had very good success with doing this.  Many oscilloscopes, even old ones, have the option of using the power line as a horizontal sweep source.
If LINE is selected for the horizontal sweep and audio from your speaker is routed to the vertical input, the display may be very telling.
Turn off any noise blanking and watch the interference spikes.  If they are stationary, your noise source is synchronized with your power line frequency.  Possibly a utility company problem.
If the interference spikes drift, it is not likely to be a utility company problem.  More likely a switching power supply or something similar.

Gary
K-zero-CX

“Another thing would be look at the audio on a scope and try to sync it 
to 60 Hz. If it syncs to 60 Hz, there's a good chance that it's power 
line noise.73, Jim K9YC”