Determining Bad Tubes

ON MY ANTHEM PRE AMP AFTER NOTICING ALOUD HUM AT LOW VOLUME I OPENED THE COVER AND WITH AN ERASER HEAD I TAPPED VERY LIGHTLY ON EACH TUBE THEY ALL MADE NOISE WHICH WENT RIGHT TO THE SPEAKERS BUT SOME WERE LOUDER THAN OTHERS ARE THEY ALL BAD OR JUST THE LOUDEST ONES. PLEASE ADVISE .THANK YOU