Hiss noise on normalisation at set time in recording

I am busy trying to finalise my third audiobook and have encountered a problem that I have not come across before. Some of my chapters are getting a brief hiss noise, lasting only a second, which appears after I have run the normalise effect. On one of my chapters it appeared at 20.59 in the recording but was not linked to the actual sound at that point. I changed the tempo of the recording and the sound came again at exactly 20.59, this time over different words. But on other chapters it appears at different times. I have tried using a different normalise programme, the RMS Normalise but while the noise sounded slightly different it came at the same point.
Any help greatly appreciated but please keep terms simple as I am not an expert in this field. I am using Windows 10 and Audacity 2.1.2

Compression, if incorrectly applied, can raise the noise-floor, causing a sudden loud hiss when there is no speech, e.g. …

Learn how to add audio to a post here … https://forum.audacityteam.org/t/how-to-attach-files-to-forum-posts/24026/1

Thanks but this is over the top of speech and seems connected to a particular time in the recording rather than the actual words as it happens at the same point even after I have changed the tempo so that it is over different words. I have also downloaded the latest version of Audacity to see if that made any difference but the sound still comes at exactly the same point.

That second sentence give an important clue. It shows that the sound is not in the track that you are editing.

You don’t mention if your project has multiple tracks or just one.
Are you working with the track height maximized to fill the screen? If so, try reducing the height of the track.
How many tracks are there in the project?

You are a genius, there was only one track in the recording that I was working on but when I did as you suggested, I found another blank window underneath. I closed that and problem solved. Thank you very much for your help as it was driving me mad!