Chances are it’s bearings related (the creaking noise my Stumpjumper is usually the bike telling me it’s missed a service). When you strip the frame you’ll probably find the bearings crunchy at best and seized at worst (bearings are designed to go around in circles really, not just backwards and forwards). Pop the dust cover off with a Stanley knife and thoroughly flush the bearings and then repack with grease. Brought mine back to life twice so far (actually second time I stripped the frame I did it before the bike started creaking and guess what none of the bearings had siezed this time). Biggest problem is if you can’t get the bearing out of the frame to access both dust covers, makes it a lot more difficult to clean out as the crud hides on the back side.