Hi guys, new here. I have a 2000 Suburban and it failed smog today due to white smoke coming out of the tailpipe. It doesn't smell like coolant smoke, more like burnt. It only happens when the car is idling for a while and the engine is revved, but after a couple of revs it disappears. The smoke is only evident at the very first rev. It doesn't happen while the car is idling either, only when it has been idling for about 5 minutes then revved. What could be causing this issue? Thanks.