Koepel:
A few ideas:It is not clear what is connected to AREF. I hope you did not connect the 3.4 V of VCC to it
Keep AREF open, but do add a capacitor of 100nF between AREF and GND. That does help.The best option depends on the sensor that you want to read. Reading a ratiometric sensor is not the same as reading a battery voltage.
Right now Aref has a capacitor to ground as prescribed in datasheet and as I guess most of arduino (mini micro, uno, etc) boards based on 328p have it can be wrong but seems like it. If I choose to provide reference voltage to Aref, then as I mentioned in original post it will be either powered from LDO or from same noisy Switching regulator via RC (or RCRC, or LC), will be testing tonight which one works better in my case.
I do not have specific sensor choice, the idea is to make clean reference voltage (without using bandgap reference) came as an interesting challenge while using analog sensors. Can be anything: mic, temperature, light meter, etc. It is actually a bit of overkill since 40mA is not bad ripple and is stable but I want to see how lower I can make it without too too much effort.