I mouth a soft grunt, and raise the volume if necessary. Be sure to have already drawn your bow; I know it sounds like a no-brainer, but I came back to camp a few years ago while bowhunting Alabama, and one of the more tenured hunters in camp had a wallhanger come through and whistled him to a stop. The buck stopped, whirled and looked straight up at him. He had no chance to draw, and lost the opportunity.