Problem with applying effects using effects rack

I'm using Audition to edit an audiobook at the moment, and one of the chapters that I'm editing has a pretty high noise floor. I have tried the noise reduction process, and adaptive noise reduction, neither of which are giving great results.

I then tried some expansion using Dynamics, by putting it in the effects rack. Before I apply the effect and I'm just previewing, it sounds like this alone has basically removed the noise, which is strange because I wasn't expecting expansion to do that much. I then apply the effect to the audio file, and it suddenly sounds much worse; you can hear the noise cutting in and out with the expander, which is kind of what I was expecting it to do.

Does anyone have any idea what is going on here? Why does the effect sound so different before it has been applied? I tried restarting the program, and I'm able to fully replicate the issue again and again. I'm getting the feeling it might be an issue with something other than Audition but right now I'm stumped.

Any suggestions would be massively appreciated!