Can't help with the rattle, haven't had a TS for a while, but the thermostat needs a bit more information.
The classic dead thermostat behaviour is
- runs cold during normal driving
- if you sit in traffic, the temperature rises to "normal"
- if you sit in traffic for long time, the fans kick in, but the temperature stays at (or just above) normal
- once you get moving again, the temperature drops back down to "cold" again
If the displayed temperature is always cold, regardless of traffic, then you have a sensor/gauge problem. If you have the behaviour described above, then you have a dead thermostat stuck in the open position.
It won't damage the engine, it will just use a bit of extra fuel. Worth getting it changed as it saves quite a bit cash per tank.