A high pitched noise which indicates the filament is stuck. I went in and made the chamfers a little better so the bowden tubes were in the Y-connector all the way. I have just tried this using PLA as the support material and what happened is interesting:
The first layer went down fine. The model pla printed and then the support pla first filled in a layer on the priming tower and then printed its first layer. When it came to the second layer, the model pla hardly moved during the tool change and then you could see it slowly travel down the bowden tube. It was so far away that nothing was going to print for the second layer. I think there is some kind of mechanical issue. I have a note in the SeeMeCNC site since the dual extrusion for my Max V3 is their baby. Any insight is appreciated though.