What causes a roaring noise in my ear (left) that sounds like my heartbeat?

Turbulent flow. More than likely you are hearing your external carotid artery. Flow in this artery often causesa roaring in the ear on the affected side. On ocassion, this artery may have to be ligated to stop this if unbearable. See an ENT or cv surgeion if not your pcp and get evaluated. You may need hearing eval as well.
Many things. There are a number of conditions that might cause symptoms like that. It could be conducted noise from the large vessels in your neck or head. It could be a condition of the inner ear or middle ear. A tumor such as a glomus tumor of the jugular or tympanic or vagal nerve could cause it. Or an aneurysm or a-v fistula might cause it. If it persists you need an ENT to examine you and get an mri/mra.