NOISY.APP  Thanks for bringing this to my attention!

I changed the random number generator from a simple mathematical formula to using the Unix /dev/random file. Although /dev/random produces more random data, it is also more computational intensive.

I can't really hear a difference between the simple formula and /dev/random, so I will change it back in the next release :)

It is also possible that the new CoreAudio APIs in Leopard are using more CPU. Let me check on that as well.  
(Version 1.2)

NOISY.APP  Noisy is based on Noise's source base. Both use CoreAudio to generate the white/pink noise.

However, the APIs used by Noise were deprecated in Leopard and replaced with new functions. Noisy could continue to use these old functions, but doing so might unexpectedly break Noisy in the future.

If you want a noise generator for Tiger, check out Noise. I believe that somebody posted a link to a Universal version.  
(Version 1.2)