A loud noise has started only when the engine has warmed up.
It appears to come from the passenger side cabin. The noise frequency is rhythmic and follows the engine speed. The car runs well and not affected by wherever the noise originates. There is no trace of the sound from under the hood or from outside the car.
The noise is masked by road noise and the sound system.

I am thinking that the sound is originating from worn components in the engine but is being simplified through the body shell of the car. To support this theory I am feeling a corresponding vibration on the passenger side floor through my fingers. The noise or equivalent vibration could be transmitted via exhaust mounting brackets and resulting in reverberation through the body shell.
If the root cause is a bad bearing I would expect to hear or feel the noise / vibration when the engine is cold. This is not the case.
Piston slap has been suggested but I find this inlikely.

I do not have the feeling that the engine is going to fail imminently but is rather suffering its age. 167,000 Kms. The car has been well maintained by the dealer and oil changes frequently carried out. The car is generally in very good condition.

I am thinking that this is probably a generic fault with the G35 because another contributor posted a sound recording of an identical nature some years ago.
The thread did not produce any answers.

Any ideas ....

 

Record it and post a video, that will help a lot more than words.

 

Having difficulty uploading sound recording.
Noise seems to clear at high revs then starts up again.
Definitely starts only after engine is warm after a cold start.

 

See if you can post to youtube as just an audio clip.