Got a new 3.5 inch HDD put in external enclosure. Is it normal to feel like a baby kicking if you rest your hand on the HDD while it is running? Happens constantly like twice a second but randomly. I have no idea as i've only ever had internal HDD before... just hope it is normal.
That will be the kickback from the heads moving across the disk. It's not normally very strong.
You'll also notice a strong "precession" effect if you turn it around in your hands.