I've taken note that some power supplies are better for use with Ardunio, than others.
What has been evident to me, is that if you are going to use a voltage converting power supply, just to power the arduino, you need to make sure the power supply can handle it.
What I mean by this, is that you need to make sure the voltage converting power supply has the bare minimum load on it, otherwise it will behave erratically, and cause errors in various types of ways.
All converting power supplies, either switchmode or linear, must have a minimum load on them, which is usually 10% of their rated current. There are some technical reasons for it, but I don't remember them. Something to do with having a load ensures proper voltage regulation and voltage output stability.
As an example, I was powering my arduino project with a 9V 1.5A switchmode power supply from an old portable DVD player. The arduino, current sensor, and LCD were drawing about 75mA. No where near what I should have been putting on the power supply. In this case, it should have been at least 150mA. Sure I could have powered a light or something, but I elected to run the project from the battery that I monitoring the voltage and current statistic from. The LCD screen no longer flickers, measurements are considerably more stable, and also there's proper grounding.
Switchmode power supplies are also inherently noisy, especially the cheap Chinese/Taiwanese ones. The switchmode carrier frequency can range from lower tens of KHz up to single digit MHz. The cheap ones also have poor noise filtering, poor RF shielding, and use cheap parts which will eventually fail sooner than later.
Linear power supplies must be loaded down with the minimum load as well, but they do not generate the RF hash like the crappy SMPS do. However, some of the linear power supplies have little or no output filtering, so you will hear the utility line frequency hum if the power supply is connected to any type of audio device. I my case, its a mobile amateur radio.
You may be able to do some software coding to filter the noise out, but this is case I'd recommend using hardware instead of software for filtering. Less to go wrong, works more reliably, and less code to be run on your arduino.