I have this weird thing going on my bike.
When I apply front brake only, I would feel a "thug" like noise under my seat. It happens at 30+ m/hr. Each time I come to a stop or slow down, when I pull just the front brake, the "thug" noise shows up, just once.
So each time I pull the front brake, there is the thug. And I let the lever go and pull again, there is the thug noise again. It does this until about below 30 m/hr.
No abs indication or any lights illuminated in the instrument panel.
But when I apply rear brake first and front brake second, its normal.

Im sorry if I make any sense. I already took it to the dealer and told them this noise. They'll let me know their findings. 2016 roadglide special at 5k miles.

My 2012 roadking never had this issue.

 

Your ABS module is under the right side cover under the seat. My guess it it has something to do with that being you feel or hear it from under your seat.

 

Sounds like the linked braking system is kicking in the rear brake. If you pull the front brake only, the system will activate the rear brake "automatically," above 20-25 mph. Likewise, if you hit the rear brake only the system will activate one side of the front brakes automatically. On my 2016 RGS, if I pull the front brake only there is a very slight delay, and then I can hear the rear brake activate at about the speed you describe. Much faster and I can't sense it. It's not very loud, but it is noticeable. The linked brakes are not active below 20-25 mph. Try pulling the front brake only below that threshold. If it does not thug, then it's probably the linked braking system you're hearing activating the rear brakes.

Let us know how it turns out.