Had the same thing happen in my 88 4ws prelude. It turned out to be loose lug nuts!

You mention it doesn't happen when you brake or accelerate. This makes sense because you are putting torque against the mag in either case. When coasting it will flop around.

Hopefully this hasn't been happening too long, or you might damage the Mag.

Take the tire wrench out of your trunk and give each bolt on each tire a tighten.