UCC25661: Is the PWM waveform HF burst mode or feedback loop oscillation?

Part Number: UCC25661

Tool/software:

Dear TI ,

I already disable burst mode by setting LL pin, but the LLC PWM looks like HF burst mode when loading increase from  0.8A  to 1.2A. (below pic.) I hear some audio noise so I don't continue to add load. 

From the waveform , output voltage 70V(CH1) is stable ,but PWM frequency(CH3)  is change with fixed HF. I see the fixed HF is the same as FB_opto(CH1).  Please help to check if  it is HF burst mode or feedback loop oscillation. How do I fix the problem ?  

CH1: FB_opto  CH2: +70Vo CH3: LLC low side PWM