2000 Buick Lesabre 2000 or 02

Are the front tires worn evenly or chopped? Does the sound change when making turns?

As far as the gas cap, there must be a leak in the EVAP system. Have the computer scanned to identify what code is stored. Most nationally recognized parts stores will do it for free. If it is a leak in the EVAP system, then I recommend having the system fogged to determine where the leak is coming from. THe mechanic will actually place fog into the system. Where there is a leak, the fog will escape. It makes it much easier to find the problem. Otherwise, you may be looking for a long time and never find it.