Hi Keith,
What is the make and model of your scope? What are you using for a probe on that? To get 20Hz waveforms like that on a wallwart is too strange! Switching types will give much higher frequency noise, and linear types can give 120 or 60Hz noise. Maybe it is some kind of behaviour happening when it is not loaded. I would load it down with a resistor and see what happens.
Be careful of wall warts. Some of the older types may be unregulated linear types. The output voltage stated is only the voltage as a result of loading, so the unloaded or very lightly loaded voltage can be quite a bit higher than listed. Also they can sometime be center negative on the plug.
Are you experimenting with DC receivers? These can be susceptable to PS noise.
73