This is a known issue with 1.4 TFSI engines installed in A3 models until 2010. The noise occurs due to a defect in the camshaft position actuator (CMP) design, which allows oil drainage during extended vehicle idle periods. During cold starts, when oil pressure is lower, the shaft rattles until oil pressure builds up.
Remove the cover and check the CMP camshaft actuator part number (Figure 1). If the part number does not have the marking 03C 109 088 E, it needs to be replaced with the modified version that bears this marking. To confirm the issue is resolved, start the engine when cold.