Isolated Modbus (RS485) ground and UART noise
Try putting a small capacitor (units to hundreds of nF) between the primary and secondary ground under your isolated DC/DC converter.

EDIT: okay so I misunderstood your schematic... the ADM2682 apparently supplies its own power for the line interface. But the advice about grounding is generic. It could be a DC/DC in your off-the-shelf 485 dongle, or in the ADM2682, or some such. General advice: use an oscilloscope, put the ground clamp of its probe to some ground potential that you deem trustworthy, and try measuring/probing other "isolated ground potentials" in your RS485 bus topology (while the bus ground is not connected yet). This way you should find your culprit. Start with the 'scope set for the fastest timebase, then gradually slow down if you find something interesting.

EDIT: another idea what to measure: attach the ground clamp of your oscilloscope probe to the power GND of your STM32. Keep your RS485 bus ground connected between your ADM2682 "isolated line interface" and the USB/485 dongle at the computer (I'll get back to that later). So you have your scope probe gorunded to the STM32 - now touch your 'scope probe tip to the ADM2862 isolated line ground ("CMN" in your schematic). Is there something interesting going on between those two grounds?

Another suggestion: in the setup where your STM32 reports "noise", attach your 'scope ground to the STM32's power GND and watch what's going on at the TTL RX wire from the ADM2682 to the STM32. If you have a dual-trace oscilloscope, you can actually watch the isolated CMN and the RX at the same time (= the last two paragraphs combined).

Another question I haven't noticed asked yet... that USB/485 dongle, I assume its grounds are not isolated (try measuring between USB ground to 485 CMN with an Ohm-meter). What sort of a computer is this attached to? Is that a desktop PC with an ATX PSU? Or, is it a notebook by any chance? If this is a notebook, does its adaptor have the secondary ground (aka DC-) connected to the Protective Earth in the wall socket? If not, the adaptor likely leaks a bit of EMI. Low current (&lt; 1 mA) but potentially high volt. If this is a notebook, the simplest test for this problem is to run in on battery for a bit (= ground floating) - and make sure that you don't have some peripheral device in the system that has its own switch-mode PSU adaptor (external disk drive or some such).

There's one topic that you should understand, and that is, why do isolated switch-mode power supplies leak EMI to the output. Can be a soft 50 Hz Voltage for AC/DC PSU's, and it can also be pure RF in DC/DC converters (same principle = parasitic capacitive coupling between RF transformer windings). I have uttered a relevant answer, with schematics, in a past topic here at stackexchange.