There are several things that can increase perceived noise in a photo:
- Increasing brightness/exposure in post processing. When you amplify the shadows you also amplify the noise in the shadows.
- Increasing saturation. The will particularly affect chrominance noise.
- Applying peripheral illumination lens correction. If you use lens correction to brighten up the corners of a photo that has light falloff in the corners you are raising the brightness levels of the corners. The noise in those darker corners will also be raised.
After editing I'm seeing a significant amount of noise that wasn't present in the original RAW image.
There's really no way to see an "original raw image." Raw data is much like a latent negative. It can wind up looking a lot of different ways depending on how we develop it and how we print from it. Luckily, for digital raw image files we don't have to make irreversible decisions like we do when developing film.
What may be happening when you first select the image you are viewing is that you are seeing the jpeg preview generated by the camera and attached to the raw file. Many raw processing applications use the attached preview jpeg to display thumbnails of raw image files. Depending on your RawTherapee settings, you may be seeing the jpeg preview image when you first open an image file. The jpeg preview image probably had an amount of noise reduction applied by the camera.
Or you may be seeing how the default options in RawTherapee interpret the data from the raw data. In any case, your monitor can not simultaneously display all of the information contained in the raw file. It has to be interpreted to be displayed on a monitor.
If you were importing your raw files into Canon's Digital Photo Professional then the application would be applying the same settings that you had set in-camera at the time the image was taken and the result will look near identical to the camera generated preview image.
When you start editing in RawTherapee many of the in-camera settings applied to the jpeg preview are ignored. RT opens your raw image and applies its default profile instead. If your default profile in RT does not include any noise reduction and you don't add any while editing then none is being applied to the image as you edit and export it. Even if you do apply some noise reduction, if it is less than what the camera applied to the jpeg preview, then your edited image will be noisier than the jpeg preview you saw when you selected the image.
For more, please see:
Why do RAW images look worse than JPEGs in editing programs?
Why do RAW images in Darktable have a lot of noise?
While shooting in RAW, do you have to post-process it to make the picture look good?
Why is there a loss of quality from camera to computer screen
Why do my images look different on my camera than when imported to my laptop?
How can I undo Canon Auto Lighting Optimizer in Lightroom? Although the question is different, this answer directly explains what you are experiencing.