2014 engine roars at startup, won't go above 1200 rpm, then clears

[BobRam]

Last few "first start up of the day", engine has a roaring sound, won't rev above 1200, sputters, has almost no power, just enough to move truck, then clears after about 5 minutes, then fine all day.
No lights or messages.
I wonder if something to do with exhaust brake, as the sound is similar.
Every morning for last week and always the same pattern.
Any ideas please ?
Bob

 

[Ozymandias]

Get AlfaOBD and check for codes.
No CEL doesn't mean no Codes present.