Engine is shaking and really loud

Tee

Hey all,

I was driving and stopped on a traffic light, suddenly my check engine light came on, rpm dipped, and the car started shaking and was really loud (sounds like a cammed V8)

The check engine light went away but the car is still really loud, rpm constantly dips, shakes heavily especially at idle.

I replaced the cat and installed a HFC and a mechanic checked for any vacuum leaks, checked MAF sensors, throttle bodies, etc.

Spark plugs were recently replaced (however one seems darker than the rest). The mechanic suspects it could be the ignition coils, but there are no codes whatsoever when we scanned the ECU.

Could it be the ignition coils or fuel injectors even though there are no codes? If not, what could it be?