Playstation 3 fan running loudly on purpose?

My PS3 (CECH-2001A) has been running loudly since I swapped out the stock hard drive for a 1 TB 7200 RPM drive. When I say loudly, it seems that the fan inside the PS3 starts kicking into high gear sooner than usual. I've had this hard drive in my system for about a year now and have played my ps3 on average I'd say the equivalent of about 4 hours a day for a year.

Just a few weeks ago I decided that it's time to go back to a 5400 RPM drive same size. So I made the swap and turned the PS3 on. It ran A LOT quieter. But after leaving it on for about 20 minutes (had to format and re-install fw), the fan kicked into gear again. This time though it is quieter than it was with the 7200 RPM. Is this just how the PS3 fan runs? I thought it would run quietly. My wife was in the bathroom and when the fan kicked on for the 5400 RPM she shouted "What was that?" I may not have minded it before I was married but, I don't want the noise to interrupt her when she is napping or sleeping.

Think on the Desktop Power Supplies that the fan only turns on when the system is drawing a lot of power from the wall (AIDA 64 Stress Test, Furmark).

Hard Disk Drives follow the rule of "higher RPM's, higher power consumption". This is because the plate is moving faster than the lower RPM drivers.

Also, a fan that is regulated by temperatures will start spinning faster because some part of the system is overheating thanks to dust and/or old thermal grease.

I would recommend a cleaning and thermal grease change for the system (If you can't do the job, find a Console Repair Shop on your city), because the 20XX systems were manufactured between 2009 and 2010 and they should be a bit dusty. Not too long time ago, I opened up my CECH-3001A and the result was a complete mess.