I think I can control both of these muscles.I can produce the low, rumbling sound by rolling my eyes up as far as I can with my eyes closed. (I can hear it when my eyes are open as well but I get a much louder sound when my eyelids are closed). But it's not just my eyes doing it - I'm aware I'm doing something else but I have no idea what it is or how I'm controlling it. It's like I mimic the very beginning of a yawn and that's how I'm getting the muscles to contract.The pop or clicking noise: I can pop my ears (my eustachian tube, whatever) at will. I do this by protruding my lower jaw forward and down at the same time. And just like with the rumbling...there's something else I'm doing and I have no idea what it is or how I'm controlling it. Again the fake yawn thing here.So there's my input.By the way...I ended up here by doing a google search on what causes the sound because I find that sometimes the sound is louder than normal and it freaks me out. During these times, I find that even the slightest contraction in my facial muscles will result in the loud rumble. I've always assumed it's when I'm stressed or nervous (this is often - I'm quite neurotic), I tend to get very tense and clench or tighten the muscles in my shoulders, my neck, my back ...and now I'm thinking...in my face. So when I barely move my facial muscles at all -because I'm all clenchy - these tiny muscles are ready to go and therefore need very little effort on my part.From googling and from reading through this thread, it seems like I don't need to get so freaked out. So thanks. And also it's kinda cool to learn that I'm among only very few people with this ability.