When the engine is cold the revs are a bit higher after starting the engine. After appr. 1 minute it drops to its normal idle revs. This is normal.
However during that first minute when cold enabling 'R' gear I hear an irregular rumbling sound (regardless of standing still or releasing the brakes). As soon as the revs drop to normal this sound is gone.
Choosing 'R' always gave a loud click sound when cold. Not sure if that's normal as well.
Car drives and shifts ok without issues...

Any ideas?