[Elecraft] KX3 tone after back to RX with SUBRX

[gt-i at gmx.net]

Hi List,
my KX3 shows new behavior; dunno since when and what I could have touched.
here is what I do: Mode CW, SUB-RX on, send a dit or dah, go back to RX, 
now there is a tone in the audio path like a "pling" which is decreasing 
in volume within about 1.5-2 secs to zero. This seems to happen only if 
there is somebody sending CW on that frequency, it is not audible if 
there is just noise. All other settings (PRE;ATT,Filter BW) don't seem 
to make that tone go away. MCU 2.76, DSP1.5
Can anybody reproduce that?
73 Gernot DF5RF