I have a 2010 Silverado with 5.3l, 6 speed tranny, 18,000 miles.

I have noticed when this truck is traveling at 40-60 mph, and it drops into the its 5th and 6th gear, I can slightly hear exhaust or lugging noise through the cab near the drivers side floor board.

I assume I am hear this due to the low rpm in those gears, since it is usually around 1000-1200RPM. Is this just a slight lugging noise? Or is this something worse like an exhaust leak?

If anyone has a simliar truck or has noticed this themselves, I would appreicate knowing.

Thanks in advance