I've owned 3 different 1800's a couple were noisier than others. My '04 was very smooth and quiet. My Gold '06 was had growling noises like you describe and my '09 growls some but not like my '06 did. In other words the noises you're hearing are common. Of the friends I ride with that have 1800's, some are quiet and a few growl like all get out. "Wolfman"
What year and model donor bike is your trike built from? I know the transmission on my 2012 GL1800 is louder or more mechanical sounding than any of the Harley transmissions I've had experience with.
Questions-
Does it happen only at slow speeds 0-30mph ?
Do you have chrome rotor covers on the front wheel ?
If so- try tightening the fittings and screws on the rotor covers. They sometimes come loose and will give of a grrrrrr type sound at slower speeds. Above 30mph they quiet down and you don't hear them.
Hope this is all it is.
That message may have hit a home run for me! My irritating noises (other than the alternator whine) occur about 30-35 mph - and I do have the chrome rotor covers. I know what I will be doing this evening!
I got my Goldwing after riding HD's for years. First thing I noticed was the weird sounds coming from the transmission, almost like it was making metal. Told it was only Goldwing normal. 8000 miles later still sounds weird, but "Goldwing Normal".