Although it does work properly (meaning the motors do rotate as they should), after the second or third rotation, they start to make a whirring electrical noise -when not moving.
I have attached a schematic of my circuit. The power supply is 5v dc 16A (old computer psu).
You can see exactly what I am experiencing on this video I made (try to listen in closely).
Update: I checked by touching the h-bridges at the same time the noise starts, and I can feel them getting really hot.