2016 Elantra rough idle
 

I've had a continuous intermittent issue with a rough idle while I'm in drive and stopped say at a light.

it seems to occur when the car's RPMs drop down to about 667. At 750 its fine. If I place the car in neutral the rough idle stops, same with park. There is no check engine light being thrown and when last checked its not misfiring.

My local hyundai deal has:

Replaced the alternator, spark plugs and checked the engine/transmission mounts

I just did a fuel system and carbon cleaning but that did not resolve the issue.

my next thought it could be an issue with the idle air control valve

Any suggestions?

One way we might see what is happening would be to look at the data stream info a good scan tool would provide.

Unfortunately , not very many places will provide that info without you paying for a diagnostic check.
If you know someone who might have that capability, some of the info we would like to see would be ;
ECT
MAF
INJ pulse width
Fuel trim, long and short
Any addl info it would provide.
Quite a few problems can be revealed with that info and depending on just which are not good we go from there.

You might look for a loose, broken vacuum line. Sometimes it would raise the idle and other times depending on where it is just provide a rough idle that goes away as RPM is increased.