RV-6 - Fuel Venting Question

Yesterday when I opened my left gas tank cap there was a brief whooshing sound.

I assumed a venting problem so I doubled over a piece of real think safety wire, twisted it up and carefully ran it up through the vent a ways and found nothing. I then hooked up a piece of plastic tubing to the vent, stood by the open tank camp and blew very gently. I could hear the air coming though in the tank. So basically I found nothing. I don't get it. Any other possible causes?

Ice somewhere in the ventline? Either condensed and then frozen water inside the tank or ice/snow under the fuse where the vent exits?

wooshing sound

How full was your tank? If it was topped off and the air at the filler cap was not adjoined with the vent line air you could get a pressure differential and a "wooshing sound" as the fuel cap was removed because the pressure of the air around the fuel cap had not been relieved by the vent line. I have had a pressure differential that was high enough to cause some fuel to be pushed out the vent (which are set up according to plans) by the pressure differential of the air trapped in the tank around the area of the filler cap.