Sentence:  The doctor sat down rather wearily , caressing the hen and remarking that the city was not the place for a poultry loving man , but no sooner was the remark out than a <WSD>knock</WSD> at this door obliged him to cover the hen with his greatcoat once more . 
Meaning of the word marked with <WSD> tags:  the sound of knocking (as on a door or in an engine or bearing)