"Oil return line"? How about the oil pressure line from the oil pump to the block? That's a definite possibility, but the first thing to do is remove the fan belt and figure out if the noise is internal or external. One that loud is bound to leave a shiny spot on the fan and whatever it's hitting (probably the crankshaft pulley) if the noise isn't coming from inside the engine. Once the belt is removed, just spinning the fan should pinpoint the problem if it's external.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!