ticking engine

I have a 1996 e320 sedan, American model, gas, 115000 miles.

I start the engine , cold in the morning. It starts fine and runs smooth. After running for 20-30 seconds, it starts to tap. This lasts for about 3-5 minutes and fades away as the engine warms.
The sound is similar to a valve/lifter noise. It also resembles an noisy injector. The noise is at the top (middle to rear) of the motor. It has no effect on performance whatsoever.
Remember-it doesnt make any noise at start up, only after running for 20-30 seconds.

Any help is appreciated.