Yes, I believe that AIO Pumps are meant to run always at a steady speed which can't be changed, I might be wrong about this though.
But, In my opinion, if the pump's speed slows down then there is less fluid circulation which will increase the temperature of the hardware very quickly. So the pump must be running steady all the time to keep removing heat from the processor.
The fans speed on the other had can be changed via software.
I read that a noisy AIO Pump can be caused by air bubbles inside the pump. One method to get rid of the trapped air inside the pump is by tilting your PC at a 45 degree angle in both directions to allow the air to escape.
See if Trixx has a Pump profile such as "Quiet" mode.
EDIT: Found this Tech site for Asetek AIO with a YouTube video showing what you can do to removing any tiny air bubbles inside the pump. This should be similar to fixing your GPU noisy pump.