rattle with acceloration
Could be a loose exhaust heat shield.
Or, it is possible that the engine is “pinging”. The OP should have his mechanic check for any loose parts on the exhaust system, and if that doesn’t yield a solution, then he should check to see whether the knock sensor is working.
2014 Elantra anti-knock/detonation sensor.
First step, check if you can wiggle the heat shield under the cat. Caution, could be very hot, best to do that when the engine is cold.
A rubber mallet or a piece of 2X4 is helpful. Fist, too. Gloves on hands always a good thing here, and eye protection.