Noise Removal Causes Warped Result

Hello. I am using Windows 7.
I am producing an audiobook.
I am taking my raw recording, which has some very low hum from the room (mostly from the computer), and attempting to do noise removal.
It doesn’t matter how low of a setting I use, I always get some light warping, or echoing, sound added to my track that was not there before.

Things I have attempted:

  1. Making sure I use a noise removal sample that is completely clean of any other sounds than the hum.
  2. Creating a foam box to place my microphone in and moving it onto a table 3 feet away from the computer, and putting a pillow against the computer.
  3. Lowering the input volume of the microphone and setting the microphone to reduce noise.
  4. Taking the noise removal sample, then compressing and equalizing, then doing the actual noise removal.
  5. All possible variations of settings in the noise removal window.
  6. Recording at midnight…

Any ideas?

It might help the ‘diagnostic boffins’ if you mention the make and type of microphone you are recording with - also, is a USB one?

Could you post a short sample of the raw, unprocessed recording that includes a few seconds of noise only and a couple of seconds of your voice.
Please send the sample as a WAV file (1 MB maximum).
See here for how to attach files: