If you're going to replace the upper plenum gasket then you should definitely do the bottom one to. The bottom plenum is actually really straight forward. You just have to fight the awkward angles to get the bottom bolts off. Legit just did this whole job in 2 hours the other week. Take a lot of pictures. It was actually really hard for me to mess up, so don't worry about that. I attacked everything from the top. Never once did I go under the car. And btw, my car was making a god awful whistle noise too. Turned out to be the gaskets. Let me know if I can send you any pictures or anything.
