hi. my zafira has done about 35000 miles and as the cold weather has arived somtimes it rattles when started cold. it is serviced and the noise sounds like it is coming from the cam cover. i remember somone telling me that some cars with hydrolic valves somtimes do this, is this true? after a few seconds of idling it goes quiet again. it runs well and is not using any oil. i am going to check to see if it's still under waranty tommorow when i go to the dealer on another matter. any info or advise will be most welcome.