In the last two weeks, when checking a certain hive, I have heard a low pitched "growl" coming from in the hive. It sounds almost like a possum or **** growl. Nothing in the hive but bees, I checked. Nothing under the hive, I checked. Its mowed around the hives so nowhere for anything to hide. Its been the same hive twice now. Its on a hive stand with several other hives and nucs. It is the last one on the row. Its a double decker 10 frame queen right lang, top entrances with queen excluders, with a super thats full of half capped and one super on top they're slowly filling. They're still bringing in a little pollen and nectar, so flow not dearth. No queen cells capped or open. They are as gentle as can be. I've torn it a part twice now looking for the cause. I haven't laid eyes on the queen, but I've got eggs and uncapped brood, I usually try to get the inspection over with quickly this time of year due to robbing concerns. All Any ideas?