It could be one of two things:
1. The turbo is operating at a lower boost level
2. you have goten used to the whine...
The turbo "whine" volume level is proportional to the boost it is creating. On a cooler day the "Next Generation controller" in our GTs, compensate with lower boost levels. this will make a lower whine.
With turbos, the only time that you have to realy worry is if you hear "Clunking" "Scraping" or "screeching" noises. Whines, Pops and flaping noises are common in turbos.
Ian