Yes, my '09 Road KIng has a very strident sound in fifth gear (like an odometer cable that has "spun out ". I took it to the dealer and the explanation was that fifth is a "helical" gear (whatever that means) and the sound is normal. It sure gets my attention. I am not 100% convinced and am going to continue my inquiries.
Try the Redline Shockproof Heavy gear oil from www.myoilshop.com . It is a 75w250 and will quiet much of the noise. Plus you can run it at least 20,000 miles as RL says you can go 30,000 with it.
The Redline Shock Proof really made a big difference for my bike. I spend a lot of time in 5th gear, so this was a big deal to me. Try it, you will not be disappointed with it.