When you stop your tow vehicle, the surge brakes are activated by the
trailer tongue telescoping into itself, and in the process, pushing a
plunger in a master cylinder which sends brake fluid to the brakes in
the wheels, thus stopping the trailer. It will stay this way, until the
brakes are released by extending the tongue back to its extended
position. When you resume towing, by putting your foot on the gas
pedal, you extend the tongue within itself, causing a clunking noise.
What I do is try to stop a bit short of an intersection, and then while
you are waiting for the light to change, ease forward a bit to take up
the "slack", releasing the brakes and extending the tongue gently.
It has nothing to do with the hitch ball.
If you are using sway bars (like a Reese Hitch) between the tongue and
tow vehicle, be careful not to put too much tension on the sway bars as
they will prevent the tongue from telescoping in on itself.

Jim

wrote:

When I released the brake in the towing vehicle and started towing, I
often heard noise coming from behind me (likely coming from the
connection between the towing vehicle and the trailer). Is it a
standard behavior of a surge brake?

I am wondering the surge brake might have engaged when I stopped the
car, and it remained engaged. When I released the brake in the towing
vehicle and stepped on the gas panel, the surge brake could not respond
fast enough to release its brake. That caused the hitch ball of the
towing vehicle to impact on the coupler of the trailer, and this made
noise. Is my understanding correct? If this was the case, I should go
easy on the gas panel when I go from stop to start the vehicle.

Any idea?

Thanks.

Jay Chan