Rattling noise while coasting
 

Can anyone tell me what this noise might be? It’s a 23 trailblazer with 24,000 miles on it. It’s previously been to the shop to replace a bad head gasket. When I take it to the dealership the service manager brushes me off and tells me everything looks good there’s nothing loose and to keep driving it, but obviously something is going on because it shouldn’t be making that noise. It happens while I’m going less than 30 mph and coasting: like my foot isn’t on the gas or the brake.