What is the reason why Outback is idling and grunting?

It may be because the mixture can't burn completely in the combustion chamber when the engine is idling, which leads to the sudden abnormal sound of the exhaust pipe, or it may be caused by too much carbon deposition in the throttle and intake. Due to too much carbon and dirt in the air inlet, the cross-sectional area of the air passage changes, which makes the control unit unable to accurately control the idle intake air volume, resulting in too rich or too thin air mixture, abnormal combustion or engine jitter, and even unable to start after starting. It is best to check the intake valve and combustion chamber for excessive carbon deposition with an endoscope. I hope my answer is helpful to you.