Oh yeah, I measured 12.3V with my multimeter.
And I am not troubleshooting any issue of mine in this post, I just jumped in to tell racecar that he was not the only one hearing the "humming sound", and that it was definitely not the siren dying on him.
The sound he hears is most probably the RCM fan. For some reason, mine is loud in the morning on a cold start, for a few minutes, and that's it for the day. It then runs at any start, any time of the day, but quietly, so you don't hear it.
All am saying is there probably is no point in focusing on that sound for the issue he is experiencing.