I can hear water sloshing around at the left hand

The 405 is renowned for having the heater matrix go bad and often it springs a leak woth an associated slooshing sound. Heater matrix is like a mini radiator which is fed by water from the engine and is located behind the dash.

It is located on the lower left hand side of the dashboard. If you look under the glove box and in towards the direction of the centre of the car you will just about see it. Check the area around passenger floor and closest to the centre console for any dampness on the carpet or signs of leaking. If this is evident you have found your problem.

One solution is replacement from breakers though companies can be found who specialise in radiator repair and if you can find one in your area this is an even better solution.

Slooshing water noise like you describe is sometimes associated with impending head gasket failure but I would check your carpets first for dampness and eliminate the heater matrix first owing to it being a known failure point on the 405.

Hope this helps.