I have a 2011 MBP 15" and I put in a 500GB 7200rpm HD. The hard drive is audible when in use. Drive runs fine and there is no issues. But I had a Macbook (late 2006) with a 320GB 7200rpm HD and when in use I still could not hear it. Is this because of the difference in case (polycarbonate vs aluminum) or is it something else? I also have an external HD (laptop size drive) and it's a 500GB 7200rpm as well. When it runs I also hear it. But the external is only used from time to time for backups. Is there any way to make the drive quieter? The original 5400rpm was silent. Internal drive is Western Digital Scorpio Black and external is Seagate.