The compressor did run at least once over a 10 minute period, I can't remember specifically when it was though.
After speaking with the dealer (who swears its normal wear because his mechanic didn't say anything about shocks, and then he said it was caused by driving on gravel !) he said he thought the fronts had been moved to the rear. After I said that maybe the problem was in the front, he wouldn't say why he thought they had recently been rotated.
Needless to say, I can't be sure if it'* a front end problem (seems less likely to me) or a rear end problem, and the dealership is too far away to keep running back and forth to get the thing looked at by an impartial mechanic.
Thanks for all the help, it'* great to find a place like this,