I would like to just say that i have had a similar problem for weeks and after much testing and pulling things about etc I think I have resolved it and can report as follows;
BMW 535D LCi 2008
Low end loss of power and responsiveness
Engine note not as nice and smooth as normal (even for a diesel)
Spool down noise from engine compartment (from turbos) when exceeding 2000 rpm but going away after
I took it to a specialist who ran diagnostics on it but 'coulndt find anything wrong with it' but did say i needed to have my swirl flaps removed.
In doing this they found a split in the EGR vacuum line so replaced this.
This did help but not that much and the above symptoms remained.
The loss of power/resopnse and spool down sound varied from day to day leaving me a bit baffled.
I took all of the boost pipework off starting at the big turbo to look for splits/cracks/leaks or worn seals - all OK aprat from the plastic adpator that connects into the large turbo compressor side see image - item 7
This was not seated into the turbo properly and was being pulled out at an angle by the securing srew to attaches the pipework to the vacuum pump front top left of engine.
A small amount of oil/air mix was escaping from here and forming on the red boost hose making it look like a boost leak.
I resolved this and adjusted the maounting bracket so that it was seated nice and square.
Thanks to Jake13 for the advice on using brake cleaner to soften the rubber grommet (item 7) to make it seal nicely.
This did not fix the problem either but I was glad to find it before it got worse and gave me further issues.
Next was to test all vac lines around turbos and EGR.
EGR was working well and opened and closed smoothly with consistent vacuum pressure readings using a hand vac and gauge.
Next was the vac hoses - these had all been replaced by an indy not long ago and after carefully removing and testing all appeared good.
I was convinced the spool down noise was the issue and whatever stopped that would likely solve my problem.
So, in desperation I removed the vac line to the large vac box that controls the turbine control valve, blocked the ends of open vac pipes/connectors and started the engine.
Revved it up and low and behold, NO spool down noise.
I introduced my vac pump/gauage into the pipework and ran her up and watched the gauge through the windscreen - it was all over the place, fluctuating up and down which i thought didnt seem right.
So, out comes the wallet (again). I replaced the pressure converter - Item 1 at the lef tof the picture.
I think this image shows the original design of PC but they were revised along with a new mounting bracket so if you are still on your originals you would probably be best replacing all for the new ones and new bracket.
After replacing this the problem was better but was not back to how it should be quite.
There is another vac line that t's off from the supply from the reservoir (item 9) and feeds vac to a little PC (item 4) that opens and closes a flap on the air side.
Ordered a new PC (item 4), fitted it on Saturday morning just gone and ever since she seems to sound much niver and smoother and now goes like a mad twat again and has put a smile back on my face so I am well pleased!!
Total amount of money spent was approx £100 - £75 on the larger PC and £25 on the smaller one.
I apologise that the above is a short story but I hope that it may help you or anyone else experiencing similar problems.