Trailer brakes make a pulsating thumping noise

[Vinman]

Title pretty much says it all.
I have a utility trailer with tandem 3,500 Dexter axles and if the trailer is plugged into the truck there is an audible “thump, thump, thump” noise coming from the brakes. The noise starts as soon I make the connection.
The brakes appear to otherwise work properly.
This has happened on my last identically equipped trailer as well and it does it with 2 different trucks hooked to it.
In both cases the noise only started after the trailers were a few years old.
Any ideas what is the cause and fix?

 

[JunkCJ6]

Sounds like the breakaway is pulled and the battery is dead. When plugged into the truck you start to charge the bad/dead breakaway and it energizes just enough to get the magnet to smack the drum, but never enough voltage to get the magnet to stick to the drum. Pull and test the breakaway battery and make sure that the breakaway is seated. Also some of the brake controllers will pulse the magnets to show connection on the controller and the magnets may be loose enough on the arms that you are hearing a thump when they are pulsed.

 

[Vinman]

Yup, like mentioned above after some digging found out it’s normal.
The truck’s brake controller sends a pulse signal to confirm the trailer is connected and if the pulse signal stops it triggers the “trailer disconnected” light in the instrument cluster.
I could have sworn the noise didn’t start until the trailer was a few years old.
I guess it’s possible when the shoes were new and still tight against the drum the noise was less apparent.

 

[Etyler2]

Have you ever manually adjusted them?

Maybe the travel is increased and is now audible.

 

[JunkCJ6]

magnet on the arm is loose or the face is worn down, shoes only move when you are moving. you are hearing the magnet hit the back side of the drum from the pulse. may be time to pull the drums and check the wear indicators on the magnets

 

[Vinman]

magnet on the arm is loose or the face is worn down, shoes only move when you are moving. you are hearing the magnet hit the back side of the drum from the pulse. may be time to pull the drums and check the wear indicators on the magnets

Yeah, the trailer is due for some love.
It’s 9 years old and never been touched mechanically. I replaced the original tires today and plan on pulling the hubs to service the bearings in a couple of weeks. I didn’t know the magnets were a wear item but it totally makes sense, will definitely give the entire assembly a good once over.
Thanks for the heads up