Engine noise and fuel consumption

[Tomi]

Run out of gas on my day at the lakes few days ago. The boat is new to me so, I don't really have a good reference but fuel consumption seemed higher that expected. The boat is Prostar 190 with RTP-1 engine. Unloaded with around 2800 rpm cruise at ~30mph I was using roughly 5 gal in an hour. Is that normal? I was rather expecting something like 3 gal/h in those running conditions.

So if not normal, where to start looking?
Engine temperature stays at 160F. Oil pressure is good, boat idles at 600 very smoothly. It seems to run well at high rpm´s too. I don't think I run it at WOT on that day at all. Plugs are all new but apart from new fluids, plugs and spark arrestor, nothing has been done for the engine.

One thing that caught my ear however, was the relatively loud rumbling noise from the engine right above idle. Is that normal?