I've had my 4x4 Xterra for a couple of weeks and have noticed something I didn't on my 4x2 Xterra. When I accelerate... up around 3000 rpms, it sounds like the engine is working hard to get up to speed. Is this normal because of the extra weight on the 4x4? I never heard this type of noise on my 4x2 Xterra. It's almost like it is straining to get there.