Originally Posted by OnTheGo
The rubbing you're hearing and feeling is normal. It's the electromagnet contacting the brake drum. It's the electrical contact that receives the feed from your brake controller.
On my Dexter axles the power for the electromagnet comes in on wires not a slip ring or a rubbing contact. On a trailer or automobile with drum or disc brakes its not uncommon for the shoes or pads to drag slightly at some point in the rotation of the wheel. This is normal.
As OnTheGo stated the electromagnet could also be dragging a bit as well and that's nothing to worry about either. It will REALLY drag when the trailer brakes are applied.
If you have a manual from Dexter that came with your trailer, read through it or visit their website. Lots of good info and diagrams on how all kinds of trailer brakes and suspension systems work.