AVF/MKB: Intake Noise - Causes & Solutions

Hello friends,

Since I woke my car (A4 B6 1.9 PD AVF) up from its winter sleep, it's unfortunately been making a rumbling noise from the intake manifold.
I immediately suspected the worst (that the camshaft had failed) and removed the valve cover.

The engine compartment is remarkably clean, with absolutely no dirt or grime, everything is spotless.
I've been driving without an EGR system for a long time, just mentioning it as a side note.
I don't think that has anything to do with it.
My suspicion is that diesel fuel might be leaking into the cylinder head through a faulty tandem pump seal and entering the combustion chamber via the northwest port, which could explain why the cylinder is so clean.

Regarding the actual topic:

The camshaft shows no signs of wear and looks excellent for a vehicle with 290,000 miles.
Unfortunately, the hydraulic lifters can only be partially seen, but they appeared to be in good condition and didn't have any holes or deep wear marks.

By the way: When the engine reaches 90 degrees and the oil is properly warmed up, the knocking sound is practically gone. It becomes unnoticeable, and the engine sounds normal.
The rumbling is clearly audible both inside the car and outside, near the vehicle. It's likely due to resonance.

What could be the reason for that?
Possibly occurring diesel fuel that prevents optimal lubrication?

I am asking for your help.

Sincerely,

Max.