I have a 2008 Silverado LT, 2WD, 5.3L, with 121,000 miles. I noticed lately that when I start it for the first time in the morning the RPMs jump up and down between 500 and 1000 RPM. It also makes a fairly loud growling or rough noise. It stops idling that way once it has run for about 5 minutes or so. The growling noise doesn't really ever go away completely. I replaced the spark plugs about 2 months ago, it ran fine after I replaced them. There are no check engine lights or anything like that coming on. Does anyone have any ideas?

 

usually egr's make start-up rough