Question???

Ok got a problem my 91 celica st has started making a knocking sound and I'm not sure what is causing it :confused:
a friend and I took off the spark plug wire boots to all the plugs and found some oil on the center two boots!
so I was just wanting too get you all's opinion on it ....I'd apreciate the help ............... thanks. :)

ooooohhhh blown head gasket, that a nice $600 fix, uinless you do it urself witch is what id do, replace the timing belt while ur at it and ur good to go