VST noise suppressor?

I've heard of some folks recording a track of just the amp noise and flipping the phase. This will cancel out the preamp noise on the guitar track

this would not work because the recorded noise is not going to have the same compressions and rarefactions as the noise in the recorded guitar track.

this will only cause severe comb filtering in the noise and quite possibly result in increased amplitude in only the noise.
 
Yeah you are right. The first suggestion would be more for amp hum (edited the original post to state this). The second suggestion would cleanup overall broadband noise, etc without investing in a noise reduction plugin.