Can't find source of echo on one track

I did a multitrack recording of a musical performance and am now in post production processing the audio.  Three songs into the project, I suddenly have echo on the track for the lead actress.  First two songs no problem, then there it is - and what's weird is that it seems to be some kind of pre-echo of the primary sound.  I have 24 tracks recorded, 12 vocal and 12 instrumental.  The instrumental mics were about 20 feet from the vocal mics and the bleed barely registers.  When I solo her track or edit the source track - no echo.  Otherwise echo.  I've listened to each channel and taken out all other vocal mics, still echo.  Disabled the smart monitoring, still have echo.  Checked all the effects, only using a little reverb on the vocal tracks, turned it all off, still echo.  

Using Adobe CC build 13.0.13.46

Any suggestions?  TIA