After I replaced the tank pump and still had the noise I replaced the main pump as well. The new main pump still has the noise problem.

I have a theory that the fuel pressure regulator is bad and not allowing enough flow through to the return line. This is in effect casuing the pump to dead head and cavitate. My reasoning is that the noise is louder at idle, when the fuel usage by the engine is minimum, and less when driving at highway speed.

I am going to try to borrow a regulator from a friend and see what happens. Has anyone had a regulator go bad and cause this type of problem? I don't have a guage to check pressure - does anyone know where to buy one that fits the volvo rail fittings?

Thanks