The rumbling is caused by two middle ear muscles. These muscles are connected to ear drum and staples, if they are activated, they make it harder for incoming air soundwave to push the eardrum, they increase eardrum acoustic impedance so less sound gets to cochlea. This is triggered by loud sounds and it serves to prevent hearing damage, its called acoustic reflex.
All muscles when activated and under tension shiver slightly, this vibration is the rumbling you hear. The reason you hear this so loud is becose the muscles are directly connected to your middle ear bones so the vibration goes directly to cochlea. You may also hear it if you tense up your jaw muscles but it will be alot quieter becose its further away from cochlea. You can also try pressing clenched fist against ear to hear this muscle rumble.
The term subsonic is confusing and vague, its generaly considered to be sound below 20 Hz but the problem is that humans can hear down to about 5 Hz if the sound is 110db loud. Also, while its true this muscle rumble does contain sub 20 Hz sound, it extends above it too, roughly to 70Hz.