1995 Pontiac Grand Prix Losing coolant & now a knocking

Engine Mechanical problem
1995 Pontiac Grand Prix 6 cyl Front Wheel Drive Automatic

Dear Sir, My sisiters pontiac has the 3100 v6. She has been losing coolant and had no heat. I replaced the thermostat because when you shut the car down you could hear the water boiling in the rad. Also the relays went bad so I fixed that. The temp never got over 210-212. After the therostat replacement we have heat and no boiling but still losing some water however not as much. Now we had been driving it for about 3-4 days to see if that would take care of it and, last night while driving at @ 20 mph taking it easy a loud knock started but it dosent sound like a rod or a like sound it is intermeinent(excuse the spelling) it appears to be in the top of the motor, where the V6-3100 is writin on the aluminum cover above the intake manifold. Also after replacing the thermostat the same type knocking started got really loud and then stopped all in about 45 secs or so. I thought then it was the water pump knocking, I dont know maybe because of an air lock, It stopped after I added water to top it off. Maybe they are seperate issues but I dont think so. Please any help would be great we cant afford to buy anything but I can repair it or we have a mechanic ay church that will do it but hes a shade tree his self. Thanks in advance and Merry Christmas!