constant base sound in background

It sounds as though you may have been using adaptive noise reduction - this is far more likely to cause this to happen than the process-based version where you take a sample of just the noise, and use that as the basis for processing. This method works best if you use multiple passes, with different FFT sizes and don't attempt to take out too much noise on each pass - just a few dB at a time works best. This also helps to keep the 'bubbly' artifacts down too. It's also worth noting that the higher FFT settings generally give better results.