If it is loose (and I suspect the top windshield molding as the source of the buzz), pull the cowl following FAQ instructions, then get some 3M butyl sealant strips to re-glue the intake screen. To keep this from falling again, tie it using small black zip-ties to the cowl grid. They are almost invisible but highly effective.

A Volvo dealer tech once told me that the buzzing was caused by the top windshield molding,and to verify it by applying electrical tape across the length of the molding.That was the problem.I repaired it by carefully raising the molding and squeezing in clear RTV.The buzzing never returned. Good luck,Bill

There are three screws that hold the cowl in place. Two very visible on each end and one in the center tucked under a wire bundle on the upper fire wall. Remove it and reglue the screen. I used RTV blobs. While the cowling is off, remove, clean up and reglue the plastic water gutter and baffle to prevent water entry to the interior. Use butyl type glue on these.

I believe it is held in place by nothing more than several blobs of that sticky black goo that is used for similar tasks throughout the car.

I would recommend securing the grill by tying it in place, with either small plastic tie-wraps or wire made of some kind of non-rusting metal. If you are clever you may be able to accomplish this without having to remove the cowl panel.