Fuel line noise on 325

[taznuv]

I've had for a while a problem with noise from my BMW E46 325 which I was concerned was the fuel pump. I asked my garage (independent) for their opinion. They had in on the ramp and say the pump sounds perfect, and think the noise is coming from the fuel lines themselves. They suggested it could be the pressure regulating valve or the non return valve in the pump, but admitted they had never heard anything quite like it. The noise is noticeable when sitting idling, sounds light and metallic, changes slightly with revs, but not directly. Almost sounds like a central heating pump when there is air in the system. You don't here it when the pump does it's initial run when turning on the ignition. I presume there were no leaks or loose fittings or they would have attended to them. Otherwise it runs really well.

Apart from starting to switch components I don't really know what to try next. Anyone heard anything similar?

many thanks