When i'm trying to fall asleep or when i hear low frequencies my eardrum feels like it's vibrating. i can hear and feel it in my ear. what is this?

Vibrating ear drum?: Sounds like fluid behind the ear drum. It could be clear serous fluid or pus. See your md.

Low frequency noise: The eardrum vibrates in response to sound wave arriving there. I would see and ENT for ear examination and hearing testing. Based on findings, recommendations can be given to to help reduce or resolve this sensation.