Growling in Benton County, Oregon

While on a hike during this nice sunny day, I heard a low grumbling sound. My husband was leading me along the trail and he did not hear this sound. I stopped him and described the sound to him as a sort of "wild boar" type sound. It was VERY hard to describe. The sound was low, deep, like a vibrating growl or snort. We looked around and listened. When we didn't hear anything for quite a few minutes, we began walking again. Not more than 3 or 4 steps down the trail we hear a rustle noise and I quickly turned to see a rock or something rather large, dark and round fall to the ground with a large THUMP! The item had landed just where I was standing only a moment before. My husband heard this large item hit the ground and we stood stunned for a moment trying to figure out where it came from. I looked around the area and found a rock about the size of a baseball sitting where the item had hit the ground. My husband and I could NOT figure out where it had come from. Was a rock in the tree and it just now fell? Did something throw that rock at us? We were a bit jumpy but kept on down the trail. About 1/4 mile along the trail we hear a loud "whomp, whomp" like wood against wood sound ahead of us up on the hill. I asked my husband what that was and he decided it was probably just some people doing work on the trail. I didn't believe that's what it was. It sounded too heavy to be man made. Maybe machinery, but there was no motor sound and it was a Sunday evening. I doubt workers were on duty at this time and on this day. Just then we heard the sound move and we turned to hear the same wood on wood "whomp" noise behind us on the trail and still above us on the hill. My husband remarked, "If that was bigfoot I don't know how he moved that quickly!" I told him that we could rule out trail workers and that it was quite possible that it was TWO bigfoot communicating with each other. We joked about it, but we were both very shaken. We just could not come up with a more reasonable explanation for this sound. My husband describes this sound as a person hitting a tree quite hard with a large log. After yet another few minutes of looking around and hoping to see something, we walked along the trail without answers. After this we heard nothing more and came across several hikers on the last mile of the trail. Several hikers had dogs and we didn't hear any dogs going wild barking at anything unusual. My husband and I were going to stop some hikers to ask if they had heard anything, but we didn't want to look crazy or scare them away from the trail. Other than the rock being thrown at us we didn't feel as though we were in any kind of danger. When I got home I called my father who is quite familiar with forest animals and noises. He couldn't explain where the rock would have come from, but he believes that the wood on wood sound could possibly be deer or elk. I doubt that it was deer or elk just because the trail was so populated. They would have been spooked by the hikers and the dogs... besides, the "whomp" sound was SO heavy and SO loud that I can't imagine a deer making such a large sound. We didn't smell anything, we didn't see footprints...but I can't think of anything else that would explain these noises and the rock being thrown at us. I just really feel as though it could have been "Bigfoot".