2007 LEXUS ES350 User Comment / Complaint

Soon after i purchased my es350 in aug 2007 i started hearing a brrrrr sound coming from the engine which is only heard in a cold start and lasts for less than 10 min. i left the car overnight at two different dealership service depts so that they would be able to duplicate the sound also from a cold start. twice i was told that the mechanics were unable to duplicate the sound which is difficult for me to believe as it is there everytime i start the car. (my husband thought that the sound was coming from the valves or tappits.) the sound was not present on the es350 loaner cars that were given to me nor is the sound heard in friends es350 cars. i am concerned that there might be a problem later on with the vehicle and that this is a warning sign.