Is your phone on Airplane Mode? If it's not, large cellular data packets will cause the same sound.
Shouldn't be anything bad, even if you specifically knew that your phone wasn't doing it before.
Automatic updates to Play Services and
ConfigUpdater can change device behavior at any time, such as cellular radios (drivers).
It could also be a new cell tower in the area causing the change in behavior. Newer cell towers are taking advantage of the latest frequencies, so that could explain a change in behavior.
If your phone is on Airplane Mode, and the sounds are appearing, you might want to check your room (and nearby rooms) for other cell phones. If you have a reason to be worried, look for bugs also.