What Causes Sudden Ringing In Head Followed By Fainting Spells?

I have had three episodes where I have ringing or a roaring in my head and then I can't stand. I go to the floor but feel as if I can't sit but have to lie down. I felt hot afterwards and nauseous. Help! I have gone through a battery of tests, EKG, heart sonogram, treadmill test, etc. no answers