Whining noise when hitting gas

[frankronne]

I have a 2012 Yaris CE hatchback with a 5MT and in the last month, when I give gas I hear a whining noise coming from the engine bay. Sometimes, it's not making the noise. It's making it in neutral and when accelerating. I told the dealership about it and when the mechanic was going to check it, it was not doing the noise (as always when you want to show something to someone...). Might it be the alternator or something like that? My car has 88 000 kms.