First of all, I apologize for not having specific 123 info for this. It's been ten years since my 123 left the barn, I have no manuals, and memory is starting to fade.

However, I can tell you that there are a only few ways that this happens. The first is that you have a partially blocked exhaust pipe. This can sometimes be fixed by taking it on the road and doing a stretch at high RPM. This is especially true if your car has a DPF, which only regenerates at high temperatures.

A more common problem is an air leak in the engine itself, usually the valve cover or the oil pan. What happens is that when the turbo is engaged, it can draw air through the valve train and crankcase. If everything is sealed up, the only thing that will pass through the vent tube are blowby gasses. But if you have a leak: a bad valve cover seal, oil filler cap seal, breather grommet, oil pan gasket, dip stick o-ring, main seals, etc, then oil-soaked air can be drawn right through the engine and into the air cleaner or turbo. Every Mercedes engine has some sort of oil separator and PCV valve, and while I can't show you what a 123 breather looks like, I've done a little writeup on the 602 which may give you some ideas:

Mercedes Valve Cover Breather

The 603/603 is a particularly poor design, because the only way to service the PCV is by replacing the valve cover. I'm not sure that's true of any other engines.

If your vacuum pump vents into the timing case, then a leak could also explain the noise, as the entire internal engine space is interconnected. So what you want to look for is any evidence that the engine envelope has been compromised.