My daughter has a 2011 Buick Regal that she bought second-hand. Not long after she bought it, It started making a strange, almost purring, shuddering noise at idle. Doesn't do it all the time, but more often than not. I advised her to take it back to the dealer but he's about 100 miles away and she never got around to it. Seems like i've heard that something like this was not uncommon in this model. Any information appreciated!