I was doing some work on wifeys Yeti and with the bonnet open it sounded like a bag of nails. Now I know diesels sound a bit like that, but this was excessive noise at idle. I put my hand on the plastic engine cover and the noise stopped.  I then unclipped the center part of the engine cover and replaced the outer section - no noise. So it appears as though the center part of the plastic engine cover can bang plastic to plastic and make a racket. Has anybody had this? I think I'll try adding some foam strip around the edges to get the inner cover to clamp down more, Or I'll just go around with the pop rivet gun to make the inner and outer plastic molding more like one piece of plastic.  What say you all?

Yep, exactly the same for me.  I used some cut up bits of bicycle inner tube to shim the gaps and stop it rattling.  I kept meaning to get it apart and sort it out properly but in the end I fixed it by replacing it.  With a Volvo XC60.

At least I'm not alone! I prised around the edge with a screwdriver and the inner part lifted away off the pegs in 5 minutes. I thought about some foam or rubber, but then thought the whole lot should be one piece. There's no problem with the whole lot wobbling about at idle because they have a thick layer of foam resting on the engine parts. I can't think why you would ever need to separate them, I didn't even know the cover was in two parts until today. I said I'd try pop rivets, but a few nuts and bolts might work. Anything to hold the inner cover hard up against the outer profile. I couldn't believe the noise was as loud as the engine. Wifey must be going deaf.

All quiet now!

I used self tapping screws in locations where there was plenty of foam thickness behind and painted the heads black. Now Yeti is as quiet as a Volvo XC60  and much cheaper to own!

I posted on here ages ago about the same thing. I found it was the engine cover location fixings rattling on the round location mounts. As above placing a hand on the cover stopped the the noise, moving the plastic bits in the cover sorted it.