keen21

Mine has (Standard GS300, Auto 1995). Look at the silver metal heater water pipes near the passenger side firewall in the engine compartment. There is one sticking vertically up with a grey plastic cap with the letter L on it. Undo it and inside there is a pin valve (like those on tyres). Press on it with a blunt object and air gushes out and you are done; my heater was not working but now works a treat after I did this 10 second job.

I had a similar thumping noise on my GS300 despite an expensive caliper and pad change; finally discovered it was the intermittently seizing lower slider pin... the easiest and cheapest thing to fix. Tigerfish has produced an excellent tutorial on this on this site...good luck.