I'm about the same age as you and just started getting a feeling just as you described. For me it's difficult to put into words as well. I usually notice it only if I'm very still, like when I first wake up and lying in bed. My heart is beating in a normal rythym but some of the beats feel "heavier" almost like I can feel the blood movement through my heart and valves in an exaggerated way. I'm going through perimenopause which may be a factor, and when I mentioned this feeling to doctors, the first thing they ask is could it just be a skip? No, I've gotten those too and this is different. I have an appointment for more testing in a few weeks.
So if you take your pulse while it's happening, what you're feeling is NOT a skip, right?
I could only guess at this one...but my guess would be that you are what is termed hyper sensitive to your heart which believe it or not is a true diagnosis meaning that you will feel things that most of us never will. What you have described is pretty much the way that it is....the blood flows thru our four valves and out into our body....so the dumping sensation may be exactly what is happening normally . If you aren't having any other cardiac related symtoms i would probably relax...i have to say tho i think all of us on this site are pretty in tune with our own bodies but i think coupled with my own history i would not want to be able to detect that in my body on top of everything else....again if you get symtoms that seem heart related you need to call your doc but i am not surprised at what you are describing....