Heres some info I posted on anther thread, not sure if this is causing your problem but its worth a shot:
There are seperate ajustments between the overall sound level, and the "background sounds" Pushing AUX1 and then repeatedly hitting the #1i.e. 1,1,1,1, raises the total sound level,AUX1- 4,4,4,4 lowers the total sound level(bell sound). However hitting AUX1-1 ,AUX-1, AUX1-1, will raise the "background sounds i.e. engine rumble etc. AUX1-4, AUX1-4, AUX1-4, will lower only the background sounds (no bell sound) If you turned the diesel engine noise down via the background sounds it will not come back up until you raise the background sounds volume. I experienced the same thing with my new k-4, I thought there was a problem and actually stumbled upon the two adjustments by accident as the directions are not very clear on the different button operation in relation to raising and lowering the different sounds.