This is a classic characteristic of a single pin V-Twin engine. With a single pin the cylinders fire right after one another (so one piston at top dead, and the other just firing down), creating that unmistakable rumbling motor. With a dual pin engine, the cylinders fire exact opposite of each other (one piston a top, one at bottom position), creating a much smoother ride. Hope I explained clearly
Not much you can do to stop this, regardless of what aftermarket mirror, bar weight, etc you buy. Just how the Single pins work. My stock and aftermarket mirrors do this, but so did my other bikes. Personally, I love it! Like to feel the power travel right thru my hands every time I twist that throttle