If known to a party or his counsel, any objections to a juror for cause shall be made before the juror is sworn in the case. After a juror has been found competent, no other or further investigation before triers or otherwise shall be had, provided that newly discovered evidence to disprove the juror's answer or to show him incompetent may be heard by the judge at any time before the prosecuting counsel submits any of his evidence in the case. If the juror is proved incompetent, the judge shall order him to withdraw from the jury and shall cause another juror to be selected.