In particular that the feedback resistors are very close to the inductor (as shown in the attached). I kind of assume EMF from the inductor effects the feedback network and results in the spikes I'm seeing?
The spikes you are seeing are happening at over 1 MHz -- so the spikes you are seeing are surely just switching noise? There's no way that the feedback resistor and DC-DC controller are responding so fast that they are creating the spikes you see. In any case, the little spikes you see are less than 20 nanoseconds long, so simple passive filtering as suggested by SaabFAN is surely the cure you need. But, why you'd
want to carefully smooth out the voltage input to a switching converter is beyond me. Save all that trouble for after the converters, even if the spikes make it through the converters, they can't be any worse. Might as well deal with the noise from your supply,
and the noise from your switching supplies, all at once -- with passive filtering and maybe linear regulation.
Citation needed? Sorry to dig up this old post, but how can this be? Put simplistically, the switching noise from the first regulator has to make it through through at least two stages of smoothing, so even if the two sources of noise are added, I would think the first source of noise is heavily attenuated.
I've seen it.
Example: 12VDC switching wall wart (typical china stuff you can get from eBay), powering a 12V to <whatever> DC-DC switching converter circuit. Scope the output of the 2nd converter and you'll see switching spikes caused by/originating from the 12VDC wall wart. Unplug the wall wart, the switching spikes disappear, and for a few seconds you have "cleaner" power until the wall wart capacitor charge runs out.
How did the amplitudes of the two sources of noise compare? That's weird. That implies the duty cycle of the second converter isn't paying much attention to the output. What was the topology of the second converter? It's quite clear that a Constant-on-time controller cannot exhibit this behaviour, nor a hysteretic controller.
The bottom line is that the second DC-DC converter hacks up whatever input voltage it gets into a massively rippley square wave. How input spikes survive the smoothing that removes all the squareness is hard to comprehend. If you're seeing poor layout allowing coupling from the input directly to the output, or common-mode noise, those are different issues entirely.