Ya, this constant hum is terribly annoying! I had the Gateway replaced because of the noise but the second one is just as loud. When the Videotron technician was here setting up the replacement, I overheard him talking to someone at Videotron about "...another one of these that's really loud…", so it's clearly a common problem. His only suggestion was to move the gateway to another room, which does not work in my case.
So, the Helix gateway is excellent at providing a strong signal throughout my condo but it sounds like a neighbour left their vacuum cleaner on and went on vacation.
I may request another replacement.