Where it comes from is every magazine article published when the S550 Mustang came out talked about the engine noise coming through the speakers on the Ecoboost Mustang. It is called the sound Symposer by Ford.
Everytime one of these threads come out there are a few folks that say they can here it but the vast majority say you have to pull the stereo fuse to kill the system before you can actually tell what the car sounds like without it. I doubt you are going to hear anything by getting closer to the speakers since it really on adds sound under load while you accelerate.
I have never been motivated to pull the fuse and I certainly could not distinguish the sounds on either my 15 or 17 Ecoboosts. On the 17 I have the FP tune which I have seen it reported that the tune turns of the Symposer but I didn't hear a difference.
Much the same with the noise cancellation that comes through the speakers. Some folks say they can tell. I have disconnected the microphone in the head liner that feeds the system and I can only tell the absence of a very low level sound when going down the road when the interior is otherwise very quiet.
I don't know about how or where these magazine articles got their info. I do know that the sound isn't pumped into the non existent speakers in my foyer where I use the remote start on my key chain to start my vehicle every morning. It surely isn't somehow wired to the picture frame on my wall that vibrates when I remote start my car sitting in the driveway out front with the windows on the house all closed. I could list plenty of scenarios that I can physically prove the sound coming from the engine is not amplified or manufactured by the car speakers.