I have a C8 with MK Flight control. Sometimes during flight I receive a low frequency beep in the earphones connected to the controller. MX20 I have audio/vario enabled and receive the ascent/decent tones but this is much lower and a single lower tone. It just occurs now and then (seems to be in level flight). It doesn't seem to be related to an error or function. Can anyone explain to me what this might be?