It's worth experimenting with settings if you use the noise reduction in Audacity- especially if it results in pulsing sounds, the attack and release settings should allow you to make that less noticeable. Also, it's better to leave a bit of the original noise than to overdo the reduction which will result in all kinds of odd, artificial artefacts in the signal (a friend of mine describes them as "space munchkins")

If you're taking a noise sample from an old tape recording, it won't be constant throughout the recording- ideally you'd want to sample from the gap between each track (or any time you have a quiet bit) and do it section by section. Sadly, there's no easy quick way to do it- it depends how much time you can spend on it, and how important the recording is.