new strange noise from my 6.0

[AC Doctor]

I have an 04 F250 6.0L that just turned 130k mi., about 2 months ago it started running rough on the first start of the day, but once reaching normal operating temps it runs fine like it always did... subsequent starts thru out the day are normal regardless of how long it sits. Yesterday, I just took a long trip towing a trailer and noticed a muffeled "pop" every 10~15 seconds. I only heard it on uphill and level cruise streatches - it does not seem to occure on down-hill runs... I kept a close eye on the temps and everything stayed in its usual position. Fuel consumption for the trip from San Diego to Vegas averaged about 16mpg pulling the trailer at 55~60mph. Today, I took the truck out and drove arround the local streets at 35~45mph and the noise was still there at accel. and cruise but not on deceleration... Any ideas?
Just wondering if I should make the trip back to SanDiego before seeing a
mechanic...