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.