My 1995 Gmc Vandura 2500 Is Making A Loud Clacking Sound ....pls Help!!

I start up the Van no problem, sounds and runs great for approximately 1.5 miles .....then a loud clacking sound begins whenever I press the accelerator. Had no previous problems until one hot summer day it died. I check under the hood and the radiator is hissing. I know nothing about cars, but could visually see the radiator resevour was low. I added some drinking water I had in the van ( Filled a little over the line) and it started right up. It would run fine for about 3 minutes or so then the clacking sound starts when pressing on the gas peddle. I just bought this van about a month ago and has been running fine until now. Only other thing I can mention is the radiator water is very rusty looking. I also do not know where the drain plug is try and flush. Yes I am such a girl lol. Any insight would be greatly appreciated. I am afraid to drive it to a shop even fearing I may do additional damage by driving it while having no idea what is wrong. It is not the wheels I believe because it runs/sounds fine until it warms up a little and only clacks when accelerating. Thanks for reading...Cindy S.