There are other things that can cause "beats". Air cycle machines, hydraulic pumps, and inverters all have characteristic whines, hums, and whistles. Even the outflow valves can produce rumbles and hums.
Some of the items are way off the basic engine frequency, but their harmonics can cause odd effects. There are so many permutations possible, many of them transient, that you can be quite entertained trying to pin them down.
Good luck