I was playing on a Minecraft Factions server (mc.snapcraft.net) and I noticed when I turned subtitles on that it said "Donkey chest equips" when there wasn't a donkey in our base. I have screenshots of this.
From where the subtitle was saying it's originating from, I'd say that it could becoming from the Mooshroom making noise, but I'm not 100% sure. It could also be perhaps the furnace or chickens downstairs.
(Sorry for the bad photo editing skills, I don't do this often)