Can someone help me diagnose this problem or where to check for a leak?

Usually when you fill up your cas tank you hear a vacum noise when opening the cap.

That's not happening for me.

 

Unless you get a check engine light (MIL), don't worry about it. Could be a fluke.

 

I don't hear a vacuum in mine either xD

 

i don't think i've ever heard a "pssh" noise when i open up my gas tank.

 

what's the cause of the "pshhhh" when you open your gas tank? pressure caused from?

 

Each type of car is different, but the premise is that as the fuel is used up, the level in the tank goes down and hence a vacuum is created if the gas cap and fuel system is completely air tight. However, this is not the case. Some air must get in or the vacuum will get too severe and hamper the fuel delivery to the engine. A bit of vacuum is needed to assist the fuel recovery system.
My S2000 has an almost unnoticeable sound when the cap is removed compared to my wife's PT Cruiser, which has a very pronounced vacuum suck when the cap is unscrewed.
Of course, it also depends on how long the vehicle has been operated. If you start it up and then go to the corner gas station, not much vacuum will have built up. Once the car is parked for a while the pressure will slowly equalize.

 

yeah, mine has never done it.

 

I only notice cars doing that when the fuel level is really low. Don't worry about it unless you get a CEL.

 

I've had mine really low and never a sound

 

I think I hear it more when it cold, also when the tank is near empty rather than refilling at half full.