I have had the same thing happen many times. To me it sounds and feels like little bubbles popping. Rubbing causes air and tears to move around in the membrane around the eye. It is very possible that your eyes maybe a little dry, so try some eye drops. Systane is a real good one. I am not an eye doctor either, but I work in an eye clinic and that's what our Doctors recommend. If at any time, you have eye pain and/or discomfort, or vision problems, see you eye doctor right away.
It's no doubt just the fluid in your eye causing the sound when you rub it. I can make my eye squeak when I rub it. I'm quite a hit at parties! I have been diagnosed with dry eye syndrome which really has nothing to do with my ability to squeak, just a "btw."
If there is ANY pain associated with this, see your eye doc immediately and stop rubbing it.
Make an appt. to see your eye doc to be safe as I am NOT a doctor.
Peace
Greenlydia