I have had this and it sounds exactly like a moth fluttering inside your ear, I usually get it when my ears are too dry
dont ask me why but I think using cotton buds can take away some of the natural 'oils' that should be in your ear and the fluttering is just small pieces of wax moving around inside your ear. Yeah I know sounds absolutely crazy
(but then arent we all!) Easy solution for me was a squirt of olive oil spray from chemist called Earol - solved the problem immediately.