Ok, given the climate down there, I'd not run 0W40, I'd run either 5W40 (what Mercedes is telling dealers to use) or 5W50 (what I use - both are on 229.3). The heat of North Georgia leads me to use 5W50, maybe a bit extreme, but that's me. It is certainly warmer where you are.

The lifters are hydraulic and use oil as their medium. If you increase the weight of the oil, the lifter(s) may be happier.

Of course, maybe it's not a lifter, I DO KNOW that our injectors are kinda noisy in these engines. But it sounds like you have one (of which ever it might be) that is way noisier than the others.

Hard to say from here.

Here is hte current approved oil list:
MB 229.3 - Multigrade engine oils (Specification 229.3) - Mercedes-Benz Specifications for Operating Fluids