Hi Bob, There is a "rumble" in the IC-756Pro, caused by improper decoupling of the +8V supply rail feeding the receive audio preamp. You may be experiencing this. Here is a link to the fix: http://www.qsl.net/ab4oj/icom/nbfix/ic756pro_nbfix.html Cheers for now, 73, Adam VA7OJ/AB4OJ Scanned for viruses by Blue Coat http://www.WinProxy.com/