i have a 92 4ZD1 truck. the head has been replaced in the recent past. 178k miles. since i got it, it has been losing coolant. i have to add a quart or so after a 20 mile trip. i recently flushed the cooling system out and the old stuff was brownish in color and looked like the original coolant. no signs of external leaks that i can find. when i shut it down, i can hear a bubbling or gurgling sound for a second or so. anybody got any ideas?