350 pop?

I have a 75 vette and the other day i started it up just to run it(i don't run it much in the winter) and it will make a popping noise every once in a while at idle.Tune up is only maybe two years old on it. I checked plugs and wires and they all appear ok. I haven't had a chance to check the timing. Could that be it?

I'd come closer to thinking it might be moisture in the distributor cap or maybe in the fuel. If the car ran alright before, the timing wouldn't change. Joe