Could the cat be gone?

[Blrob]

When driving down the street, I can hear a popping sound sound from underneath the truck, around the cat. I don't smell anything burning, it also seems not to have good pick-up vs.before the noise started. The noise mainly happens when accelerating, not standing still. Also, I noticed the oil gage is fluctuating between the "A" in normal and going all the way to the left, like when the engine is turned off. The oil level is good, what could cause this?