I bought a 2005 330i couple weeks ago as a new project car, it had this knock when I first bought it and plan on fixing if it seems worthy. The car has 137,000 miles, runs and shifts perfect but the knocking is constant and doesnt affect driveability what so ever. It does get faster as the RPMs increase. Also squeaky belt was resolved as I spilled coolant by accident. It is more profound as the car warms up.

Videos:
https://youtu.be/4auknHsaVHw
https://youtu.be/buiz_7reFAU

So far as just new car maintenance goes I have done:

Spark plugs

Cold Weather CCV

Intake Manifold Gaskets

Throttle body gaskets

VCG

Radiator

Expansion Tank

Water Pump Pulley

Hard Cooling Lines

Exhaust Camshaft Position Sensor, OEM(old wire was just bare)

All Vacuum Lines

Fuel Pump

I was thinking it seems louder than a bad lifter, maybe a rod bearing? I looked at videos with stuck lifters and doesnt sound as deep as this. Also the previous owner says its had this knock when they bought it.
If any more information is needed I'll gladly help.