'knocking' ?? could just be a spun bearing cap...
if caught early enough, before major damage is done, cheap fix, slide new bearing in.
But of course , if it has a lot of hard hours on it, maybe remove and totally rebuild ?? THAT will cost big bucks...and there's the 'while it's apart'.....problem... new rad, hoses, stat, inj pump rebuild, new wiring, paint,etc...