Is this your first Hill style bow? If so, understand they will never be dead in the hand like a recurve or bow with significant reflex deflex. The design and physics won't allow it. A Hill bow is supposed to have a decent "pulse" to it, but a well built one like Chuck's ought not rattle your teeth. I can't stress enough how important it is to grip the bow firmly, like you would a bird or a gerbil so it won't get away but without killing it LOL. With 15gpp any Hill bow ought to be a pussycat. When gripping a Hill bow you need to make sure you're putting even pressure along the grip with the heel of your hand, otherwise the limb timing will be off and THAT can also contribute significantly to the so called thump. They really are brilliant sticks, they will telegraph immediately if something is off about the shot which makes it easy to learn consistency with them.
I dunno. If changing how you hold and shoot the bow isn't doing it then maybe that Echo just hates BCY X, but I find that hard to believe. Could it be a spine issue and the arrow slamming into the riser? This can make the shot loud and you'd definitely feel it too. You could try a skinny D10, but I wouldn't hold my breath on it removing more thump. Also, one more thing, too many twists in a Flemish will increase thump in my experience...I try not to have more than one twist per inch max at my desired brace height.