I often hear a pulsating/swishing sound in the back of my head (has happened for over a year). could this indicate an aneurism or other serious issue?

Unlikely to be: anything more than wax build up in your ears but certainly if it worries you I would suggest you make an appointment with a health care provider to give you a thorough evaluation. He/She will want to know more about your family history and will want to examine you. Unlikely you will need testing if a thorough examination is made.