The brief
The A4 was leaving fresh oil drips around the sump area, and the level was dropping enough to need a top-up every couple of weeks. The owner had also caught a faint knock at idle on the occasions the oil ran low, which is the engine telling you it would rather not be doing that.
A slow oil leak is the kind of thing it is tempting to live with by just topping up. But the source needs finding, because a top-up habit can hide the day the level drops far enough to do real damage, and that faint knock was a preview of it.
The diagnosis
On the lift, the leak traced cleanly to the sump pan itself, a stress crack near one of the mounting points. That is a common ageing failure on these pressed-steel pans: heat cycles and road vibration work a fatigue crack at a stress point over time.
The gasket was actually still doing its job, holding the joint sealed. The crack was the leak path. So this was not a re-seal job, where you drop the pan and fit a new gasket. It was a pan replacement, because a cracked pan will keep leaking no matter how good the gasket is.
The work
Drained the engine oil, supported the engine, and dropped the failed sump pan. Cleaned the block face back to bare metal, since any leftover gasket material there would stop the new pan sealing flat.
Fitted a new VAG-spec sump pan with a fresh gasket and new bolts, torqued in the correct pattern so the pan pulls down evenly against the block. Carried the oil level sensor over to the new pan.
Refilled with the correct oil spec, then held the engine at idle and checked underneath for any weep before the car went out. A leak fix you do not verify under running pressure is a leak fix you might be doing again next week.
The outcome
No drips overnight. Oil level holding cleanly between checks. No idle knock.
The A4 went home with the underside dry and the slow loss closed off. For the owner, that means no more top-up habit, no more oil spots on the driveway, and an engine that always has its full charge of oil rather than running low between visits.
Catching the crack at the drip-and-faint-knock stage kept it to a pan swap rather than the much bigger bill that comes if an engine is run seriously low on oil.