Remember, this is a Atkinson
Remember, this is a Atkinson 4 cylinder with low RPM torque and higher RPM(s) are required under heavy engine loads. That said, Ford increased the horsepower from the older 2.3L FEH to the new 2.5L Atkinson FEH. Ford also changed the exhaust system and the engine sounds louder than my '05 FEH in certain conditions. The '09 and '10 FEH will drain the HV battery lower because they go EV so much easier and all the time under 40mph in the '09 and 44mph in the '10 FEH. The '10 FEH also has an electric A/C compressor that will run during the A/C and defrost mode. What this all adds up to is the engine is constantly under a heavy load from the generator charging a low battery. My '05 even gets louder under this heavy engine load when the battery is at its lower limits.
You could have figured out how to drive in EV more after 1800 miles and therefore the engine will be louder if that is the case. One way to tell is when in the morning after a cold start-up the engine goes through a warm-up procedure and uses the electric motors for most of the torque (without a heavy foot on the accelerator) for acceleration. This drains the battery and the engine does not use the generator as much for charging till after the warm-up procedure is over (about 1/2 mile). If the engine starts getting louder like your hearing after the warm-up, this is normal. If the noise is even loud during the warm-up procedure also, you may have a problem.