blaidd,
as you've probably worked out from Fourwinds post, leaving offerings is not encouraged in these parts. the general consensus seems to be to leave these sites as you find them.
that aside, hope you enjoy HH and TMA
Peace and Noise,
Squid
There are a variety of opinions on the offerings issue. I'm not too offended by the biodegradeable variety myself, but I know there are a lot of people who just don't like stuff left around at sites.
As for vibrations, that's a strange one. There weren't any pile drivers in the area? Could be so many things I suppose. If in doubt, I like to blame the farmers.