I dunno what you call this technique but sometimes there's a very dirty way to get unwanted sounds partly out, but the piece you're sampling from needs to be in stereo. Now if the sound you want is in stereo then the unwanted sounds need to be mono, or vise-versa.
Sound Forge has a channel conversion effect ("Use difference between channels") that will remove most of the mono and leave you with the "stereo difference" (in mono. if the orignal was a single sound processed into stereo, you can process it back again if you want to.)
I guess what it does is apply (100% mix) one inverted channel to the other non-inverted channel, and vise-versa, cancelling any mono sound. So if it's the mono sound you want, you can apply that stereo difference as-is (it's already inverted) to each channel in the original and at least one channel might be left with mainly the mono sound. You might not notice the artifacts once it's in your mix anyway.