Sounds like the heater matrix leaking; the
gurgling sound is air in the cooling system due to the leak. Check behind the centre consul, just behind the radio to see if the carpets are wet. Not a quick fix I’m afraid, nearly all the dash will need to be removed and more. Se this post
http://www.toyota-supra.info/forums/showthread.php?t=8495&highlight=heater+matrix Check out
mrnickleye`s link.
The heater matrix is made from polypropylene and similar to the radiator. Air is blown from the heater fan through the matrix; the air is heated and then goes into the air vents. Not sure but I think Toyota do an exchange unit, probably expensive though. Plenty of supras on ebay being scraped you may find one there. Or there is a supra parts dealer in
Somerset who may have used one if you need it, that’s if Exeter is in the UK.