This took entirely too long to figure out:

Ground noise – 24 VDC wall wart – probe on gnd lug

That’s with the scope probe ground clip connected to the wall wart coax connector barrel and the scope probe tip on the ground clip. It’s not the noise on the 24 VDC supply, it’s the noise injected into the ground connection!

Huh. Makes it tough to sort out low-level signals, it does indeed.

Consider one of my bench power supplies at 24 V:

Ground noise – bench supply 24 V – probe on gnd lug

Nice & quiet, the way power should be. One might quibble about the residual noise, but at least it’s not blasting out horrific bursts at 120 Hz.

For completeness, the PCB inside the offending SMAKN 24 V wall wart:

SMAKN 24 VDC wart – PCB

“High Quality Commercial Grade” my aching eyeballs.

[Update: Edits based on eagle-eyed observations in the comments. ]

Not as many missing components as I expected, though, if the truth be told. The missing transformer common-mode choke seems odd and, AFAICT, the resistor inductor angling out from the R1 callout doesn’t connect to anything, connects directly to the AC line because  C5 is missing and the pad joining them doesn’t go anywhere else it replaces the jumper (?) to the bottom-left pad and the missing parts. The red LED in the upper right isn’t visible through the black case, although it might serve as a voltage regulator.

Over on the far right, beyond the transformer and between the two capacitor cans, is a component marked C9 with an oddly angled part. Seen from the other end, it’s a ferrite bead:

SMAKN 24 VDC wart – output ferrite

I don’t know why that spot has an inductor symbol with a capacitor part callout.

The other side of the PCB looks clean:

SMAKN 24 VDC wart – PCB solder side

It’ll probably serve well in a noise-tolerant application, maybe an LED power supply.

As pointed out in the comments, there’s a UL mark:

SMAKN 24 VDC wart – label

Not sure what I’ll replace it with, although a small 24 V power supply brick may suffice.