Could even be rear mech hanger, I try and keep my shock bushings lubed with a squirt of GT85, same with under seat rails and the bits where it moulds into seat base.
Just had all my pivots out, lubed with grease and then nipped up any play.
One tip I discovered by accident
When tightening up the shock bolts wind them in and when near to taking up slack swap from front to rear mount bolts gently taking up any play an eigth of a turn at a time until the last bit where I nip up one by a mm then other etc, if you wind in one and nip it up then tother all way in you “might” get the chain stays mis aligned and this will squeak, it will also blow your shock. “Apparently” ahem.
You may know this but I didnt