[Mordi]

Hey guys.......lately i've been hearing a boom sound from my fuel tank, my tank is expanding and contracting. The problem is i cannot locate the main issue, but i guess there's something wrong with my tank's ventilation system. Btw i own an i30 1.6 petrol 2009 model ......i also tried leaving the fuel cap open for a day and the sound disappeared. The sound only comes when the car is running! Please if anyone knws a solution to that problem...share!!

[Dazzler]

That is another odd one... 

I would suggest a visit to your dealer but i'd bet money they either won't hear it or can't fix it 

[eye30]

When you remove the cap do you hear a whoosh sound?

If so, then there is a vacum created.

If not I'll still take Daz's advice and get to dealer to ask them to sort out...

[Dazzler]

When you remove the cap do you hear a whoosh sound?

If so, then there is a vacum created.

Very good point Lester 

[Phil №❶]

Disagree, Isn't the fuel tank under slight pressure due to excess fuel return from the injectors. This is normal & as the fuel cap is non vented when the cap is opened the tank pressure escapes.

If a true vacuum was developing, the car would eventually stop due to fuel starvation, even though there would be fuel in the tank. This is not the problem that you describe, so I don't think there is a vacuum at all. 

No fuel tank vents to atmosphere anymore. All fuel tanks vent to a carbon filter/canister - even when the car is turned off. When the car starts  - the hydrocarbons that have been collected in the carbon filter get sucked back into the engine to be burnt during the combustion cycle. There are one way valves etc in this system.
As previously suggested, after running the engine - remove the filler cap, if you hear a whoosh as atmospheric air rushes in to fill the void created by the used fuel then something has gone wrong with a valve/filter in the emissions system. All systems follow the same principles but will be subtly different - over to the dealer to sort out.

[Phil №❶]

Instroe

If my theory that the tank is pressurized, you could check in calm conditions, holding a feather (not a candle or cigarette smoke) to see if i]air is sucked in or forced out. Don't let the neighbours see you, or they'll think your crazy.