Relay Chatter on 6M when the ATU is pretuned and enabled

With the ATU engaged on 6M USB, there is a rapid relay chatter at about 20-30Hz on voice peaks when transmitting. Using the tune button gives the same result above about 6 watts or so. This sounds like RF feedback into a control circuit, but only is SSB modes. If the ATU is left on and then you switch to CW the chatter does not occur, even with full power. I'm not sure if this relates to the reported CW tune problem or not. If the ATU is in bypass mode no chatter is seen in any mode. Tests were performed both on a good 6M antenna and dummy load.