I had a very similar experience to Charlie Stephen's on Meall Horn in the Reay Forest, Sutherland on August 24th, 1970. It would have been around midday: the weather seems to have been identical, as I recall it being a perfectly cloudless and windless day.
I was sitting contentedly in the sunshine among the summit rocks next to the cairn, relaxing after eating my lunch in the perfect silence you only get in the hills and looking towards the adjacent peaks of Arkle and Foinaven, when I distinctly heard a noise coming from beneath the summit rocks. It lasted a few seconds and I can only describe it as being a cross between rushing wind and rustling leaves in a hollow space and it seemed to move around the cairn. As the day was so markedly quiet and wind-free, I was disturbed from my slumbers and, being unable to identify the cause other than attributing it to the unlikely phenomenon of a sudden localised gust of wind, I left the summit in an apprehensive frame of mind.
AIl I would add is that, having climbed extensively in the north-west Highlands over the past 40 years and often in pretty adverse conditions, Meall Horn remains the only mountain in Sutherland that I have climbed just the once.

Roger Galt