So I bought a new gpu and one of the fans is making a "brrrr"/rattling noise. When i put my pc on the side (so the GPU is vertical) the noise instantly stops. Do I need to replace the fan or can i just lube it and fix it that way?
When you say "new" is that new to you? or brand spanking new?
Also this seems like a possible bearing failure in one of the fans, do not apply any lubricant to the fans, you don't want that flicking all over the GPU.
The only possible thing you could try is to blow the fans out with compressed air to see if there is any dust causing this issue, if not, send back under warranty if possible.