I have a 5V usb power bank battery that I'm trying to charge with a 5V dc adapter rated for 2A. The adapter is a very cheap, generic one.
I cut the end of the usb charging cable and soldered the vcc and gnd cables to the respective pins on a female dc power jack. The third pin on the power jack is not connected to anything. When I connect the 5V dc adapter to the power jack, which then connects to the charging port of the power bank, everything works and charges as intended. However, there is a feint humming/buzzing noise that comes from both the adapter and the battery.
When I power an arduino with the same adapter, there is no humming. I think I also noticed the sound when powering a solenoid.
Searching through Google, it seems like it's okay, but this can't possibly be a good thing. Is it a matter of getting a better adapter? Or am I doing something fundamentally wrong?