Why is my engine making a whining noise when I accelerate

There's a lot missing here that we need to better help you, but the first things I do with a whistling noise depends on if it's a NA or turbo motor. if it's NA, a lot of the time that's something like a pulley wearing out or a bearing that is worn inside the engine somewhere - if it's just the belt pulleys, you can usually tell as the noise will be where the pulleys live on the front of the engine -- for example if they are on the side, you will hear it on the passenger side or if it's on the front. It will be sharply in the front and is somewhat accessible, but might be tricky. The other common cause is turbos -- but if you do not have the WRX STI (2L EJ20), it's a NA engine. The other common items which are not severe are the alternator pulley is going out.

If you hear it coming from inside the engine, it might be something like a timing part inside the engine that needs to be looked at, and that will be a properly extensive teardown. The power steering pump requires a partial teardown.

ALSO: If you can do it, change the head gasket - especially if you have an EJ-20 or EJ-25 and the engine is being pulled out to do it. These engines (the others like the EJ15/16 aren't known by name, but the EJ20/25 is infamous) are infamous for needing them done at the worst times, every single time. If you have to do a pulley and they are too cramped without an engine out pull, you might as well do it now and get it over with. YOU WANT THIS ONE: 11044AA770 for the EJ20, 11044AA642 (NA)/11044AA770 (Turbo) for the EJ25 -- this is the newer MLS part which doesn't fail. The EJ25 from 2011+ is fine, but anything from 2010 and older is known for it.