I have an 04 limited 6 speed manual XF that is a little bit rough sounding at idle.
When I'm at a stop light or just pulled into the garage and shift out of gear and let off of the clutch, then I hear a faint noise almost like knocking, but when I depress the clutch it's smooth as butter.
I did do a sanity check and tried this with the emergency brake engaged and then with it disengaged, same result both times.
There's no fluctuation in rpm idle, it stays right between 5k and 6k rpm.
I'll be replacing the air filters this evening with K&N.
Does anyone know what this could be and is it something to worry about.

Any help with this would be greatly appreciated.

Update: K&N air filters installed, oil changed (mobil 1 - 0w 40), K&N oil filter installed.
engine is running smoother than ever(while driving), but still have the noise when I'm stopped with the car out of gear and the clutch disengaged.