A low hum in Grays Harbor County, Washington

My husband was driving while we were heading north up the WA coast from our little town of Ocean Shores to Moclips, for me to shoot some photos. I keep my cameras handy while we are driving almost anywhere out of our town. I like to shoot candid photos of the sights we see. On June 2, 2012, we reached a curve just south of Seabrook (a newly built, planned community). My husband pulled over and I turned to look up at the rather bare fir trees standing on the top of the ridge on the east side of the coast road. I always enjoy looking at those weather bleached trees and have captured their a number of times. When my eyes moved down to the lower part of the trees, just uphill from our parking spot, I saw a head and shoulder-like shape standing between two trees. The brush on the hillside hide all but the head and top of part of the shoulder shapes... I quickly reached for my 'ready to shoot' camera, with its 70-300mm tele-lens, but by the time I aimed the camera at that spot...the head/shoulder shape was no longer 'there'. I told my husband what I'd just seen...but he didn't seem too impressed. So, I stepped out of our SUV to shoot some photos looking downhill and out over the beach below our rather high vantage point. I shot several, including some of wildflowers blooming nearby my husband's driver-side door. When I was about to walk back around the car to get into it...a sudden loud noise struck nearby. I immediately looked toward the wide-metal guard-rail, since the loud noise sounded like a heavy stone hitting metal...leaving a vibrating sound. I saw no stones on the paved parking area...but a stone could have skipped over into the grass and brush just beyond the railing. I don't know what caused that loud noise...but it was unnerving enough for my husband, who does have a hearing problem, to suddenly tell me to get back into the car. He had heard it also. While first parked in that area...we had also seen a large pickup truck suddenly stop just below us right on the curve of the highway...while it was traveling north, as had we been just minutes earlier. The truck was not sitting in a safe place...it we noted that it made no sense for it to stop in such a blind-spot on a curved in a busy roadway... The truck had then moved forward 20 to 30 ft. and then suddenly stopped a second time...and hesitated for several seconds before starting on its way once more. Later...my husband and I speculated that the truck might also have been struck by a rock/stone thrown from that same hillside... Our rock incident was the second one I know of that occurred within the same week... I had been told about and shown the broken windshield on a lady's car just days earlier...when a rock had been hurled up, over and out of heavy brush while she drove alone to Aberdeen, Wa from Ocean Shores...passing a swampy area near the Humptulips River.