My roommate just came home and pulled me out to the parking lot. His truck is making a pulsing noise coming from the radiator and coolant recovery tank. Every second it makes a loud "woosh" noise. But it doesn't happen when the truck is running, although it overheats quickly.

Any thoughts on what might be causing this?

 

Fat air bubble in the system?? What year truck? Tons of miles?

 

Got it in the garage and shined some light on it. A piece of the fan snapped off and lodged itself in the radiator. Thanks 99silver6.0.