Am having a backfireing or popping sound coming from my 06 tahoe, always at road speed and always intermittent. Sometimes when desellarating, sometimes when uneven roads are experienced, sometimes for no reason at all, sound varies from one "pop" to a dozen In a row like a machine gun. Hard to catch it and have time to do anything like drop the accelerator or accelerate to see if a change occurs. The best I've done is to glance at the tach to see if that changes during the popping, but so far can't seem to see the tach affected. Changed plugs and wires and checked wheel bearings to see if they were "piling" up, but that checked out tight and plugs didn't change anything. Any body got any ideas?