I finally been able to look at the data from when John removed the etherant yesterday. During the entire 5-hour period, the speeds from 7m are higher than those at 8m. Thus, the difference is not due to flow around the etherant. (I had suspected that this would be the case several days ago when I noted that the speed difference (all u-component) was not a function of wind direction.)
John and Khoung are now swapping the 7m & 8m sonics to diagnose this problem further.