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 Super Air 210TE
Previous : 2000 X-Star, 3000lbs ballast, LTR 330, Acme 1235 14.5X14.25prop. 1200 hrs of flawless fun