I could feel the throbbing in his chest as his heart beat strongly.

throbbing 是动词 throb 的现在分词形式。throb 的意思是"（心脏等）有规律地强烈跳动、搏动"，通常用来描述心跳或脉搏的跳动。 throbbing 在这里作名词用，指"（心脏）持续而有力的跳动感"。