Yep, this is not an uncommon occurance. They often follow tractors, and have even been known to hop up onto 4 wheelers left running. Some suggest the thump of the motor is similar to their drumming. Im not sure about that, as many who run a chain saw get the same reaction from grouse. I have heard of many folks being "befriended" by ruffed grouse. Its just something that some of them do.