MacBook Pro: 7200 RPM hard drives may vibrate
If you have a hard drive that runs at the 7200 RPM speed, you may sometimes feel a vibration when you touch your computer.
These drives have a faster platter rotation speed than the standard 5400 RPM hard drives. Due to the faster rotational speeds, you may notice a vibration when the hard drive accesses data. This is normal behavior.
Learn more about hard drive noise.