I'm driving a 2013 Chevy Tahoe with about 179,000 miles on it, and recently it's been overheating. The engine temperature gauge keeps rising and then goes back to the middle. This only happens when I'm driving, not during startup. There's no check engine light or warning messages, and the AC and heat work fine, but the fans are really loud. I don't think I need to add coolant because there's no low coolant message. Should I check the coolant level? I'm thinking it might be the thermostat, but I want to be sure.