Microcontroller power supply
Cheap multimeters will read DC as approximately double on an AC range. Almost certainly, if your multimeter is cheap it will do this. Try measuring a battery and see what you get....

When measuring AC voltages, the signal is rectified and averaged to "create" a DC value. The circuit that converts an analogue signal to a digital number works with steady DC values and the rectifier and averager circuit provide this. So, if an AC signal were 10V RMS, after rectification and averaging it would yield a DC value of about half i.e. 5V.

If you fed 5V DC into your meter (on AC ranges), it would be rectified and smoothed and would still be 5V. So now you have 10 VRMS and 5 VDC both being processed to the same value. Of course, on AC, the clever little circuits know that they need to compensate the numbers they have read and, in effect, they double the numbers to make AC readings accurate but this has the effect of making you think that there are serious levels of AC on a perfectly stable DC supply.