It sounds like ETD indeed, and the `glue' is probably mucus or water. Do your sinuses feel full? Did you have this for a long time or did it start recently? If your ENT says your ears are fine, that should at least rule out any infections behind the ear drum.
My ears also pop and crackle while yawning and swallowing, while looking fine through the otoscope. I can never breathe freely through my nose and sometimes one side is closed up completely. Nasal sprays, steaming, salt water, exercise... nothing seems to help.
My ENT said that this all is normal for some people, but I doubt it.