Know what you mean Martin. I remember a couple of trips to the rollright stones, and had the feeling was being 'watched', it's rather a strange place. Have spent nights at circles alone, and never so much as felt threatened by a whisker. But the Rollright's have a 'darkness' that maybe to do with the pine trees there, or maybe not ;-)
Silbury gave me a start a few years back, again the feeling of being watched, yet this was more the sense of an 'intelligence', it was twilight, and I hurried on by.
In honesty, I more often find seemingly innocuous places that scare the pants off me, a particular wood, or field, or copse, valley etc. The stones usually have some kind of 'regulated' feel that doesn't terrify as much as, say, 'Cannock Chase'. Never worked that place out, but ask anyone about it and they may well shudder. (I tried this last week and it worked).
Nothing to do with Moon Cat's rellies either, I warrant ;-)
Another one was just a lane, in Buckland In The Moor (Dartmoor), there was a woman, in white..no..really!
Those barrows be the worst I tell 'ee ;-)
RhagggeeeE!