There is a way that I used to recycle amiga samples.
Best case is that the sample itsself has a quiet part containing only the noise. So you could simply use a noise reduction routine, take the noise as a profile and remove it with FFT calculations. Unfortunately in 8Bit samples the noise only appears when sound is there. Simply because the low bit amount causes the noise. So if you want to have the noise as a profile, you’ll have to generate 8Bit noise without having useful sound in it. Here we go:
Start with an 8Bit sample.
Generate a subbass sine (something about 80 Hz if you like).
Turn that 8Bit sample into a 16Bit sample (otherwise you’ll never get the noise out).
Take an EQ and notch the subbass, so the noise remains.
Take this noise as a profile.
Turn your 8Bit samples into 16Bit ones.
Use the noise profile on your noisy samples.
If you lost some brightness on your samples, use an EQ to push the highs after denoising them.
You should keep one thing in mind (you might know already):
You have to decide the threshold of the noise. Though in 8Bit samples the S/N-ratio is about 48dB, it doesn’t mean that a loud dark bass sample is noiseless or that a quiet high hat is noisy (by ear). I used to fiddle around with the noise reduction’s parameters to help me out.
Hope it helped. Wish you some success!
Edit: While we’re on it, I’ll simply mention some more ways. Noise reduction is fun (sometimes)
You can also take the original 8Bit sample, turn it into 16Bits, take a quiet part of it, kill the useful sound of that part with an EQ so that the noise remains and take this one as a profile. This might even work better because the colour of 8Bit noise slightly changes with the source sound.
Aaaand there is a very simple way left for certain samples (basslines, bassdrums), mostly samples that begin with some higher frequencies and end up with low frequecies. Take an 8Bit bassdrum (just an example). It begins with a kick (relatively high frequencies) and ends with some bass. The noise actually will only be noted where there should no high frequencies be left. So you take the filter envelope in Renoise (or anywhere else) and play Mr. Dolby B NR on your own. Initially the cutoff is open and then you close it following the sample’s flow.
Okay. I admit that’s all stuff you might already know but now it’s written down either for caring or not caring