Hi.
It's worth a look on the Techlib website, particularly here
http://www.techlib.com/electronics/VLFwhistle.htm There are a number of different circuits there.
I built the VLF receiver featured in Practical Electronics, October 1979 but haven't tested it yet as I need to build the associated aerial. I will get around to that hopefully this year. PE magazines can be found on the ARH website.
Another interesting project which I hope to build was in EPE magazine, April 2003. That was an Atmospherics Monitor. This can again be found on ARH.
Regards,
Symon
EDIT: MM beat me to it with the Techlib link