After hopefully a quick piece of advice.

I have a 2004 XC90 2,4D with 126k on the clock. A couple of months ago I got a thorough service on the car, and mentioned to the mechanic there was a rattle coming from the underside of the engine. Service was carried out, and the mechanic fixed the rattle. His words as far as I remember went along the lines of:

"When I removed the oil sump, there was a plate held in by 3 screws, 2 of them were missing, so i tightened the last one up and the rattle stopped."

Indeed the rattle did stop, for about 3 months, and now it has returned. Presumably the remaining screw has loosened once more.

Need to take the car into the garage as I have a broken drop link, and was hopeful that the above description would help me describe the issue to the new mechanic so it looked like I knew what I was talking about.

Any ideas what this phantom plate with missing screws is?

Thanks in advance, not really an engine aficionado in any measure.