Bob K, I have to disagree. The things “lost” in an AAC stream (or MP3, I won’t address other methods) are not too likely to mitigate noise in an original, if anything, it’s more likely it will push the noise floor UP and make it more audible. Remember the “throws stuff out” is actually adding noise to the original.
I have heard a misbegotten codec remove some background noise, but let me tell you, the piano fade into that missing noise was “quaint”, and that’s a euphemism.
This has to be the original provided to the FLAC (or other lossless) codec. There are many, they all have very close to exactly the same rate, and that’s because the mathematics is pretty obvious to signal processing types, and the entropy can be estimated to start with.