Trying to locate a noise on an '07 BMW 335i. Twin turbo/DI N54 3.0L, 123K. I'd almost think it's some of the turbo piping come loose, but it doesn't fluctuate quite like I think the turbos should. Not directly related to engine, vehicle, or cooling fan speed, either - sounds like it's coming from toward the back of the engine. Car has decent power but seems to get gradually weaker above 2,500 RPM (I'd think the TT six should really be hauling tail about that time) and the limp mode light (symbol of an engine sliced in half) comes on when it gets up close to 4,000. No CEL. Ideas?

https://youtu.be/Nf7Kugf9CxQ
https://youtu.be/uwu1ZYcwTnM

Boost leak, wastegates failling or out of adjustment, vacuum lines going to vac canisters could be broken.

Check for boost/vacuum leaks first, but I suspect it's a wastegate issue...

VW uses something called an "N75 Valve" so that the computer can open and close the wastegate or VNT nozzle with turbo boost pressure. I am sure you BMW uses something identical, but is named differently.

When the engine is off boost, spray carb cleaner around the intake manifold. If the intake gasket has failed, some of the carb cleaner will get sucked in and change the way the engine runs. It is a common problem on the Dodge SRT-4.

There was a huge recall on these early ones for wastegate issues, Id check to make sure everything has been updated.