CW sidetone problem

6500 running V1.8.4.84, Windows SSDR

Now that I've mostly (ha!) finished writing code for my controller, I've started using my Flex 6500 on air. I notice that on the first dit/dah of each transmission period there is a quite noticeable rasping sound on the CW sidetone that lasts for perhaps 50-100ms. Investigation reveals that it is actually the "tail" of the receive audio continuing into the start of transmission. If I turn the active slice's AF gain down then the sidetone is pure.

The effect is more noticeable with short break-in delays, because the RX is recovering more often, i.e. in between individual letters or even each dit/dah. If the delay is set to, say, 500ms, then the effect mostly goes away because the RX is only coming back on in between words.

I'm using the powered LS audio output on the back of the 6000. I hear the same effect on the headphone output. The effect is rather unpleasant, especially at CW speeds around 30WPM and with fairly fast break in (20ms or so).

I'm surprised by this as all the reports I've seen suggest that the CW break-in (and, by inference sidetone) behaviour is excellent. Has anyone else come across it?

73, John, G3WGV