When I start the engine, I get this loud revving/rattling sound for a few minutes. This is a new occurrence and has never happened before. When I step on the gas, it occurs again at low speeds and goes away above 25 mph.
Could be a loose heat shield somewhere around the exhaust system. They typically happen at a certain RPM and not all the time. If you don’t want to crawl around under and hit and shake the exhaust system parts, I’d suggest taking it to a local (not chain) exhaust shop and ask them to look it over.