My 2000 502 mag wistles pretty good at cruising speed.Would a K&N filter kit quiet this thing down?

Probably not. This is a common complaint that was present on 454's, 496's, and 502's. It's the airflow flowing over the throttle body blades. Mercruiser combated this by putting shaped plastic pieces on their 496 throttle blades in about 2006 for this reason.

You can also make a plastic shield to put around your intake if you get creative. Keep it at least a couple inches from your flame arrestor so it doesn't restrict airflow. I've seen them made before that have insulation on the backside to help absorb some of the sound. Just make sure it doesn't get pulled to the flame arrestor by the air being sucked into the engine.