Wonderful recording, full of 'atmosphere' and sense of being in a vast space - well done! I well know the Byrd Mass in Four Parts, from which this really haunting Agnus Dei is the final bit. Sure, it isn't a professional quality performance, but nowadays I would be more inclined to listen to recordings like this repeatedly rather than a polished professional performance commercially released, and with no real 'atmosphere' or sense of your being in a real place on a real occasion. I haven't listened to my CD of the Mass for decades now, and probably won't again (will probably soon go to Oxfam round the corner!) :-)
From the other electrical noises on the recording it sounds like the mains-hum was being picked up via induction, https://en.wikipedia.org/wiki/Electromagnetic_interference rather than sound being picked-up via the microphone. Putting the recorder in a kitchen foil wrapper may help in such situations to shield the wiring from the EMI , taking care not to short-circuit anything with the aluminium foil.
I just received a cleaned version of the recording by Yuval. Thanks for that and thanks Timbre for the tip. The source of the 50Hz noise was an enormous OLD transformer (5kW) hanging in the tower. Yuval I will upload the cleaned version of you soon. Thanks again.
Great sound, thanks! this one's also part of the Hell soundscape for a talk bed on my radio show tonight, doing a Halloween special. Here's the blog entry where I put all the Freesound attributions, and tell you where and when you can listen. http://www.theglobalvoice.info/blog/?p=645 After first airing, it will only remain in the Gallery for one week before replaced by the next week's show, so let me know if you didn't get it.