2005 Chevy Uplander Mini Van (grinding sound)

I was recently working on a 2005 Chevy Uplander and whilst it was running I noticed a periodical grinding noise from the rear of the car about the fuel tank area, because this area I found to be difficult to find problems and actually see them I wasn’t able to diagnose anything. The dealership was made aware after I told the owner and they said that it was an air pump for the suspension, and that when there is excess weight in the rear I will do this. I drove the vehicle by myself with no cargo and the noise still occurs as well as when there is seven people and cargo in the trunk space. Every no and then it will have a false start, like turn over you think that it starts but no enough to keep running this has occur to several of the drivers of the car. I am not sure if I believe what the dealer had said, they said that they had replaced the air hoses in an attempt to fix it, and it still does the same thing. Any info or common problems for this vehicle would be appreciated as it is still is under warranty and can be fixed. Thanks for any suggestions