1999 GMC Jimmy Running Hot

The anti freeze is not disappearing it has to be going somewhere. You must have a small leak. Check under and around your car for any puddles. If there are no puddles then you may have a more serious problem. The antifreeze could be leaking through a head gasket, crack in the block, or cracked head. First thing to do is check your oil and see if any antifreeze has seeped into it. It will look milky and whiteish. Then check yoour antifreeze to see if there is oil in it. This is easier to see as the oil will float on top. Sometimes this condition is accompanied by white smoke coming out of the tail pipe. If either of these is the case then you need to head to a repair shop. As for the noise you are hearing it is very possible that it is an exhaust system issue. A basic exhaust system is made up of four parts. The Engine pipe, the catalytic convertor, the intermediary pipe, the muffler and the tail pipe. If any of these parts are worn (which they do rust and wear) it will cause a noise under your car.