when my MDX reaches around 50mph it switches into 5'th gear and the engine switches into a lower frequency tone, I can hear the low frequency tone only when I accelerate. And if I put the gear in D4(so that it won't go into the 5'th gear) then I won't be able to hear the low frequency tone. Personally, I believe this should not be a problem, but just want to get some comments. Thanks.