The brief
A BMW came in with a disturbing noise from the engine, a whistling or hissing, and the owner had been told elsewhere that the whole valve cover needed replacing, a big bill on these. He came to us because that didn't sound right. The valve cover on these has the crankcase breather, the PCV, built into it, a valve and a diaphragm that manage the pressure in the crankcase. When that diaphragm splits, air gets drawn through the tear and you get a whistling or hissing noise, sometimes a rough idle with it. The whole valve cover is expensive, but the breather part of it can often be repaired or that section serviced rather than replacing the entire cover, so the right move is to fix what's actually failed.
The diagnosis
The check traced the disturbing noise to the crankcase breather valve built into the valve cover, the diaphragm split so air was being drawn through, which is the whistle. The rest of the valve cover and the engine were sound, no need to replace the whole cover. That's a repair of the failed breather valve, a fraction of a complete valve cover, rather than the big replacement that had been quoted.
The work
The valve cover was attended to, the failed crankcase breather valve repaired or its part replaced with genuine BMW-spec components, the diaphragm sorted so it holds, and the cover refitted and torqued back down in sequence to the manual figures, sealing flat. The crankcase ventilation hoses were checked while it was apart. A road test confirmed the whistling noise gone, a steady idle, smooth running, no oil leaks, and no warning lights.
The outcome
No whistling noise, a steady idle, smooth running, a dry valve cover, and no warning lights, for far less than a whole new cover. The car went home with the noise fixed properly. The disturbing noise was the breather valve, not the whole cover, so repairing what had actually failed sorted it at a fraction of what had been quoted, which is the difference between a fair repair and an over-spec one.