Is it using water, if the answer is no it's not the matrix.
It's very unlikely to be the matrix causing it to have water in the rear footwell, there are various areas that leak, but check the obvious, make sure the door drains are clear, and if all else fails do the water test with the seats/crapet removed.
A fave place at the front is the bonnet pull cable, where it passes through the bulkhead there is a gap between the cable and the rather lagre and probably perished grommet around it, water comes inhere and drips off brake pedal, quiet a lot of water can get in here, but nowhere near as much as you are getting.
You don't mention it using water, it it were the heater matrix you would of course be losing water at a hell of a rate to fill the footwells, and the car would be contantly steaming up and spewing steam once warm from the heater vents, they do when they are that far gone, ask any vauxhall owner.
The noise you hear is most likely just the fan motor for the heater, but very easy to replace, or even oil, my old sierra I put 2 drops of oil on the broznze bush bearing of the motor every six months or so, lasted forever, they usually start with s screaming noise that last longer and longer, but 15 minute job to replace.
The matrix if gone is not a hard job at all, a bit long winded perhaps, but doable by any competent diy er.
tabetha