With the above mentioned the MCX makes kind of like jet engine noise around the 15mph mark. Same happens with my LTR when I start off. I can't recall exactly what makes it, but if I recall correctly its the fuel getting to engine or something of that sort. Perfectly normal.
CURRENT: 2008 SANTE 210
2000 Mastercraft Xstar
2004,2005 Ski Centurion Avalanche C4
2003, Ski Centurion Cyclone
99,2000,2001, 2002 Ski Centurion Elite V/Hurricane/lightning
88 2001 Nautique