Hello~I am glad that you are going to a chiropractor, they can help is so many areas, especially those involved with the neck and upper spine. I would certainly continue with him. I don't want to scare you, but maybe it would also be advantageous to talk to your PCP to rule out the possibility of an aneurysm of your brain. It probably isn't, but the pulsating feelings can be due to one.
I know that concussions can certainly cause this problem, but I would not think it would be the reason now after 3 years, plus, your chiropractor has been working on your neck, so, I doubt if this is the reason. I also know that tension headaches can present them-self as throbbing or pulsating, so maybe this is another type of tension headache.
There are lots of reasons why you can get this pulse feeling in your head (odd it is in just one position though . . . is it in your forehead ?) The concussion isn't recent so would be surprised if that was the reason. A simple neck alignment issue can cause this pulse though -- so for reasons like that all the way to serious health concerns. I would personally see your medical doctor to discuss this. Chiropractors are good but you know . . . they are not doctors. They didn't go to med school and probably should not delve into the area of diagnosing things like this. Do you have a regular doctor you can go to?