there are 2 instances in the game where noise changing will occurr naturally:
1. after meeting dark phantom on the astrowave and fighting the viruses you noise change for the first time into a random noise form. (story driven)
2. you get a warning message on your hunter vg and you have to fight some g viruses. it is very likely that you will change into another random noise. (not story driven)
if after several instances of scenario 2 you still cant get corvus noise, then go out, buy and action replay, and look on the internet for codes that will change megaman into the noise you want him to be. (youtube would be a good place to look)