4 inch batch rocket bath

Hi team my rocket bath heating fine but now winter is close temperatures are dropping. Once the system is warm ( I heat the flue with a small 3 inch steel bean can rocket to get the draught going) I get a horrible pulsing in the Firebox had it in summer also just not so severe. When burning hardwood it is not as bad but when burning pinus radiata  ( soft and very resinous) it pulses. Do I need to open my primary air supply more or is something more sinister afoot. My csa is about 10% higher than Peter recommends but I figure that by heating the flue via an outside source during startup should overcome that the same way a bypass does it had to be something to do with fuel type/ air intake.

Cheers for any advice
Russ the kiwi nutter