Crown or onlay.

Hello. I have been experiencing a heartbeat in one of my molar teeth. Some time ago there was a large filling put into this tooth  and from he onset I could say I could feel this tooth. I would not hurt, sometimes I would experience discomfort and heartbeat under it especially while exercising. I told that to my dentist who took a number of X-rays and could see some blurred spots but could not say for sure that it reached the root. Anyways, he referred me to other dentist for root canal procedure and advised that a crown would be needed after root canal procedure. I had a pre-determination done by my insurance and saw that also a post and core procedure would be done along with root canal. I had another dentist look at my tooth and she could not say for sure either whether I would need a root canal or not. She said a Cat Scan of my tooth (not covered by my insurance) will tell me exactly if i need root canal done or not. She also said that i could go with onlay procedure if so I decide. So now I do not know what to do.  In case I need a root canal done, would a crown or onlay be a better option ? would I need post and core if I decide to go with onlay ? If cat Scan determines that I do not need root canal , what can I then do about heartbeat? I read about pulp capping, would that be an option?