From a different forum, an interesting bit of information that might be useful should you run across this condition.
Question to the forum:
"Hi guys, hope everyone is doing well. I'll get right to my question. My 2000 Road King sounds terrible when I first start it after trailering. There is just under 17K on the odometer. I'm the second owner. Engine is bone stock. Oil filters from Harley. It will knock like there isn't a drop of oil in it when it first take it out the trailer. It will eventually stop. It's fine every time I ride it until we revisit the trailer again. Any suggestions (other than don't trailer it) would be much appreciated."
Best answer after some guesses and comments:
"Trailiered lifters - you're trailering it in gear, it rocks and the rear wheel slightly moves the trans internals which slightly moves the crank - just enough to cause bleed down on a lifter(s) and collapses it. I answered this call about a million times on the tech line at every rally. Stop trailering it in gear.
Starting it a few times in a row always seemed to help pump up the lifter faster than just riding it."
Good information to know along with putting your bike in transport mode.