I did a good deal of rat hunting, and it seems most of them pop in the area between the pond, digs, ruins and crypt.
And I definitely agree with the previous poster that the whisps are PHes. I went and starting killing whisps from the pond towards the ruins, and before I had finished clearing everything (mostly whisps at that point) I had rat pops near the pond.
Edited, Nov 24th 2008 3:50am by SteveInJapan