You beat me to my next suggestion - giving the pump itself a little love tap with something solid.
The variable speed pumps are relatively new (4-5 years) and seem to still have their bugs.
Mine was replaced under warranty a couple months ago. It was never completely shutting off, so it was running at a very low speed until it overheated and the thermal protection cut out.
Been using 12 volt water pumps for well over 25 years, mostly in boats. The only way I found to really quiet it down was to add a 110 AC well pump into the system for when on the power grid. That works really well on our boat where we are almost always on 110 power at the dock or running the genset.
I don't need to add anymore weight to the trailer, so I don't think I'll be doing this upgrade yet
On the trailer, both this one and my previous Sunnybrook TT, I wrapped every line I could find with the foam pipe insulation from the hardware store.
Especially look for any areas where the water lines are against a floor, bulkhead etc.
If you understand how the pump works you will realize that there just is no way of quieting it down and still keeping it cost effective.
I should add, as mentioned above - they aren't perfect". I have a spare in my basement that I would take with us on boat trips. Wasn't that big a deal on the boat, because of the 110 back up, but we liked to "dry camp" on the boat too (no genny).