For all early 850 owners, and maybe late models: For a year and a half I have been driving around with a hollow popping noise that sounded like it was coming from the right front just under the hood where the strut mount was. After replacing the mount ( which needed to be done anyway) the sound persisted. Then one day I emptied the contents of my back seat into the floor board due to a brake slam because of the idiot in front of me. After digging out all my stuff from under the drivers seat I noticed an open vent under the seat. I stuck my hand in to make sure nothing had gotten in it, and sure enough I felt and eventually pulled out a golf ball that had lodge in there from a previous brake slam. Finally, no noise!! Any strange noises in your Volvo? Check the vent, ya never know.