my 156 2,5 had approx 76652km when I had this problem.
To provoke sound: with windows down, start up and engage 1 gear, let go of clutch and let it roll without touching throttle. Then just touch throttle (0,ish sec) and let go to let engine brake, then touch throttle again. If this slight braking and accelerating makes a slight tick or rattling and metallic sound as if there is something loose.
To provoke sound 2: just as the clutch begins to engage (as you let go off the clutch pedal) and if you then depress the clutch to disengage. Just let the clutch plates touch each other and then depress the clutch to disengage them. This will also provoke the sound.
I hope this helps you to find the cause! And that the cause is a bottle in the footwell...because of the price of repair...
Best of luck!!!