03 Deville. Valves rattling
 

I have 160000 on a DHS. I've notice lately. With the gas pedal partially pressed down and around 30-35mph. The valves rattle until I press further down and the car picks up speed. Not sure if it's not getting enough fuel or the tranny not down shifting at that speed?

Codes? Carbon build up is a possible cause.

The only code is 0420. I've replaced the CAT and all the sensors. I didn't have this problem until I had to replace the Phelum. I sprayed the intake and then the code started.

I would review plenum repair. EGR would cause it but should set code. Carbon build up seems to be mostly likely cause - either on pistons or within EGR system.

I will check the repair. How would I rid the carbon at the piston?