Fuel syphoning back into the tank noise?

[bass1175]

I was ideling today when I heard the sound of fluid coming from the rear, I traced it back to the fuel tank and when i opened my fuel cap I heard what sounds like fuel syphoning back into the tank. I never heard this before and was wondering if this is normal. It was very audible since I heard it while I was sitting in the driver seat. Is this normal or is this an indication of something going wrong. It sounded like bubbling and fluid going back into the tank with air. I used to have pressure build up when I open the cap and now I dont. Do I have a clogged vent somewhere and where can I look if thats the case? thanks

[Merc600sec]

injector return ..with low fuel level you might hear it...

[bass1175]

Thanks merc, thats what I thought but was skeptical since I never heard it before. It was pretty loud, not sure why I never heard it before. Does anyone else hear this? Has to be pretty quite with a half a tank and engine running at idle.