I have a feather stream and also get the rumblingMy long-keeler went from being on rails to exactly what you describe upon - fitting a featherstream! At full speed holding the wheel is hard work.
I also get a great deal of rumbling from the prop area that increases with speed that no-one seems able to explain.
That'll be as much down to fouling the prop as it is the hull, and it's just not working as well. Not sure where you're going in the second part!A further thought. When my hull is very clean at high speed 7 - 7.5kts you can really feel the wash over the rudder. After a few weeks of fouling it is much less pronounced although I no longer get 7,5kts then. It also kicks to port in astern. However shaft drive boat b