The early V8's on the first model year of the full-size Toyota Tundra have a similar knocking noise, and it was determined by the factory to be piston slap. It disappears once the engine warms up. I wonder if it's the same for your car.
By the way, Toyota says the piston slap is harmless and is not replacing or fixing engines that are affected.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".