ugm2000

Dear forum

I am just new here, but already very impressed on the professional postings and the answers and suggestions!
I just excuse myself beforehand if I disturb this highest level with my first post ....

The initial problem is, that I own a 530XD, E60, 2006, 140.000 KM and I intend to keep driving it, because the interieur leather seats are so luxury ....

I need some advise for a systematiy failure search for following problem:

Cold engine and car running fine! After 20 minutes highway at 80mph all ok at approx 2500rpm. After stopping at first traffic light (in town) car makes strange sound from the bottom of the engine/transmission area (right side) if increasing the gas (sounds a bit like in the beginning of worn out connecting rod bearings).

The first garage (around the corner) told me that it can be the turbocharger wheel, or the connecting rod bearings or everything else on the engine. Messages from OBD: 4203 faulty, IBS faulty, 4B10. They suggested to first change all injectors, then look after the turbo charger and then continue to dismantle the engine.
After that I went to a BMW garage: They replaced all covers below and above the engine and examined the sound with a stethoscope and came to the conclusion that it comes from the transmission.

I tested the following: gear in position P and N and heavily increased the gas - no noise! Gear in R or in D and gas increase - noise! This indicates to me that it might be something
with the transmission!

BMW garage found out, that my tire dealer installed the wrong tires (225/45 instead of 225/50) - does this harm the X-Drive?

Does anybody of you has heard about such an issue and some hints, what to check next before dismantling the transmission, X-Drive etc.?

Hope I described the situation precise enough ...

Thanks in advance
Uwe