[hiccup]

Sounds like you have a leak in your freshwater tank! Typically, Tiara used welded aluminum tanks for freshwater, and they can pit to the point they develop holes over 20+ years in service. They aren't typically installed in a way that makes them easy to replace, although I am not familiar with the location of the tank in the 3500. I know in the 3800, the teak & holly sole needs to be cut in order to replace the freshwater tank. Fill the tank with the hose and listen around the boat for a tank that is getting filled with water.

There is a small chance there is just a leak at the pump fitting and it's siphoning the contents of the tank down, but this is not likely.