I have been having problems with my car heater for some time. I have tried lots of coolant and it has not worked. When I turn the car heater to the highest setting it does not work for a good 15/20 minutes (it simply blows out cold air). After around 20 minutes there is a crunching/grinding sound coming from the vents on the passenger side and the heater suddenly starts to work blowing out very hot air. This lasts for differing amounts of time and then the temperature warning light comes on. After a while the warning light goes off (the car heater is on the highest setting the whole time). This pattern is repeated over and over. I would be extremely grateful for any advice. Many thanks.