Hi Don, The IC-756Pro "rumble" is a known problem; the cause is leakage of the noise-blanker drive signal, via the +8V supply rail, into low-level receiver AF stages. There is a specific fix: http://www.qsl.net/ab4oj/icom/nbfix/ic756pro_nbfix.html Best 73, Adam, VA7OJ/AB4OJ North Vancouver, BC, Canada http://www.qsl.net/ab4oj/