I've read it's somewhat normal, but I recently starting hearing it. What do you think? It's only a slight whistle which gets stronger once I get closer to the tank.

 

I've never heard of such a thing. The only thing I can imagine is a partially blocked vent tube, but even that doesn't make sense.

 

I read it could be the fuel pump which is supposed to be in the tank. Could be a sign of a bad fuel pump. But not sure how to check if it's in the tank ...

 

That would makes sense I suppose. One way to check is to measure output. There should be a minimum amount of fuel output from the pump by volume over a given timeframe. A manual for your particular model would tell the specific values. I don't have a clue as to what they are.