LT3757 SEPIC Configuration Low Input Voltage AC waveforms

Hello, I have a design using the LT3757 as a SEPIC converter for a power supply board.  The LT3757 takes an input voltage that can vary from 10.5V up to ~17V, and converts to 12V @ 6A.  When I draw full load and decrease the input voltage below ~11-12V, I start to see an audible AC waveform appear on the input voltage and current.  At a certain point as the voltage decrease to ~10.5V the system goes critical and all my voltages and current spike and saturate on the input side. I've observed this behavior both off of battery supply and direct power supply.  The output voltage stays very stable until I get to ~10V, then it begins decreasing. (Current is blue channel 3, input voltage is yellow channel 1, green is output voltage channel 2). I was hoping someone might be able to explain what is happening in this converter?