Yes like the gentleman said it possible it could be the Gas Cap is Loose, Also the Rubber seal around gas Cap is hard or cracked Not Sealing Completely.. Also its possible it's a Vapor Emission hose that Crack & letting Vapors Escape?? Or bad check valve that's not closing Completely.. Vapor leaks are real touchie & time consuming to find the Problem.. You have basically 5 items that should be checked…Wish you best of Luck!! DANNY Mechanic