how loud (in decibels) is the average 7200rpm hdd?

Solution
Perhaps 20 dBA@1m.
Source is spcr review of some hard drives; a good source if you value quiet:
http://www.silentpcreview.com/article29-page2.html

Hybrid drives have some value in a laptop where you might be limited to a single device.
Unfortunately, the benefit of the nand cache is not great unless it is large enough to hold most of what you use.

A SSD is silent and performs much better.