2018 Chevrolet Sonic 1.8 l
161,000 MI

My girlfriend has a 2018 Chevrolet Sonic 1.8 l. The maintenance on the car has been poor. History of sludge buildup as a result of neglect. Car operates okay but engine seems to have almost a hollow like, foghorn like sound to it. It's not smooth running at all. Car previously jerked and lurched but changing the ignition coils and plugs helped that. I assigned the purge valve as the likely problem for the rough running and consistent hard start after putting gas in the car. Changed the purge valve but the problem persists. The 1.8 l has the built-in pcv valve into the valve cover. Am considering looking at replacing valve cover thinking that maybe it's the PCV? The sludge buildup was pretty pronounced due to girlfriend not changing the oil in 20,000 plus miles so I tend to go with an association there in some way as to these problems. I've looked at vacuum leaks can't hear or see anything there. Any thoughts on what could be causing this problem?