What Causes Brain Lift And Rumble Feeling In The Back Of The Head?

Hi, I am having on symptoms. About one year ago I started to have odd sensations which I considered to be in the back of my brain. It felt as though my brain would lift and then rumble (without noises). It has been increasing over the last couple of months and is happening daily now. I just had an MRI and it was considered normal. My eeg was abnormal but the ironic thing was I did not feel the sensation until the tech had me hyperventilate. The two abnormal rhythms did not occur during that time. I also have pvcs and am medicated for this. My carotid arteries were checked due to a bruit and the ultrasound stated they were clear. Any ideas? Kathryn