Good Honda Goldwing Techs are hard to find anywhere let alone Michigan. Most dealers do not sell enough quantity of wings to be exposed to unusual noises. If I was to get an honest evaluation I would first describe when the "growling" first began to rise its ugly head. What do you do to get it to growl speed, gear, RPM etc. Also if you can duplicate it every time and can get a good sound recording. Try posting it here on the forum with as much information as you can. Keep in mind there are few guys on here that are very sharp and my guess is that they have experienced the same anomaly. I personally have not heard any growling on any wings that I have owned or worked on or came through a shop where I worked to give a hint. But that does not mean someone else has not heard or experienced it. Too bad I leave in the Thumb where you could drop by on Saturday and I could listen to it and maybe give a hint as to where it might be located.
Start with some basic information and see if Fred H, Cycledude and others can start a remote diagnoses session to get you pointed in the right direction.
PS. I just remembered about 7 years ago we had a trike come in the shop for a growling grinding noise. It turned out to be a drive shaft u-joint going bad. It would do it just pushing the bike but worse when in gear and making a turn (slow speed). I cannot remember what brand of trike it was but he called the manufacturer of the trike kit and they sent him a new drive shaft assy. Just for giggles reach under and check the shaft for excessive play.