I know EXACTLY the sound you mean too SJC.
Sadly we don't hear it at our local anymore since Air NZ and Qantas pulled their 767s out of here.
But I noticed the same sound last month when we had a BA 777 visit with RR Trent engines.
I'm trying to get the same sound on my Flight Simulator 767s but haven't found it yet.
I've never heard that "howl" (or hum?) on any other type; mind you, we don't get much here (NZWN).
We're just a small international (more like regional) airport.