I got ahold of a project from another user that is having this issue and have some answers for you. It took a bit of digging but here's what I found out:
There is a bug with noise removal in CS8 that causes it to not be as effective as it was in 7. This is being addressed. However, this is NOT the cause of the hum being added when noise removal is enabled.
The noise removal components we use in CS8 will add hum to the audio if the incoming signal has a DC offset. This is most often caused by faulty audio hardware. It's easy to see it if you put the audio into an editor like Audacity (the waveform won't be centered at zero).
It's possible that the components we used in CS7 (which were different) automatically corrected DC offset. We weren't aware that the new ones do not do this. In the long term, we will probably be exploring a way to do that ourselves. For now, the only way will involve splitting the audio out to a separate file and using something like Audacity to correct the DC offset. I may post a quick tutorial showing how that can be done later on. If you can get as far as Audacity, the Normalize function (under effects) has an option for DC offset.
Sorry again for all the trouble, but thank you for reporting the issue.