1) Retighten EVERYTHING. I had a screw fall out one of my X/Y joints one time because parts had settled and I didn’t take the time to check tightness. I would put a wrench on literally every screw on the printer that I could easily access and check for tightness. Especially those on the gantry!
2) Check all of the belt tensions.
3) Check belt alignment/tracking. If you are getting “rubber dust” by any of the idlers, this is a sign that you are getting rubbing and are wearing your belts prematurely. (The silver discoloration of belts by idlers is normal. Don’t worry about that!)
4) Check grease on the rails. There should be a thin film of grease, but you shouldn’t have globs in the corners. If you do have globs, clean them up. If your rails are completely dry, then that’s pretty bad and you may need to check if your rails have been damaged. More than likely, if you lubed properly during your build, then you should be fine.
5) Check wires in cable trays and the tool head for signs of wear.
6) Clean the interior surfaces, especially panels, to get any styrene buildup off. Residual styrene can be a pretty major cause of smelly printers during printing!