Back firing 2.7 4 cyl

I have a 2008 tacoma 4x4, 2.7, 5 spd that has 11k miles. In the last 8k I have put on truck I have experienced 6 events where a pop out of the exhaust can be heard. It has happened while at speed and not shifting or pulling a grade and once while wife driving.
After the second time I mentioned it to dealer when in for first oil change and inspection etc. He said that there was no evidence on the "memory bank" or codes. He also said that there were no bulletins issued. It just happened again today at 70 mph on a flat straight hiway stretch. No one has messed with ignition or washed down engine.
Any ideas?

 

Have you tried running a few tanks of Premium fuel? Might be old gas, or water laden methanol gas. The alcohol absorbs water and could be the issue. I always run premium.