I call their bluff. Has traffic increased? Beyond doubt, but it was not that light "22 years ago".
I'm curious.... is there a website of archival timetables for the GO service 22 years ago?
It would help confirm veracity of claims --
If it was hourly, did not run as late, and the trains passed each other almost at the same time, that could be almost an hour of peace and quiet between the pair of hourly opposite-direction trains passing. And more sleep, too.
Now if it's half hourly, the trains pass each other at very different times, you may have only 14 minutes respite between trains for a much longer span of the day. You know how your sleep feels if you woke up every 15 minutes (Half-hourly, opposite-direction trains out-of-phase of each other) rather than every ~55 minutes (Hourly, two trains passes each other almost simultaneously).
Unlikely secondary factor: If there's now an infill station nearby that did not exist 22 years ago, then there's much-louder acceleration noises for two trains an hour accelerating out of the nearby station... Especially if it's using a track closer to the house. And if engines were newer 22 years ago pulling shorter trains, that may be additional factor too. All combined, can be far more annoying at only twice the train traffic. But we'd need comparative info to confirm.
Depending on the data, a noisewall may actually be highly justified, as they are going up all the time throughout the GO network, and many more will also go up between now and GO electrification.
This may be plain old NIMBY, and patience needed for inevitable scheduled noisewall installations coming during GO RER construction (tree clearing needed for 4th track resulting in noisewall installs becoming simultaneously necessary & easier), but historic data exists and a verification check may be possible/reasonable (timetables, stations, locomotive fleet, most commonly used track) to determine best phasing/prioritization of specific noisewall installs...