Also check the saddle itself. Often they creak where the top meets the rails.
If it isn't the saddle or post, it might be the rear hub shifting slightly in the right dropout under higher chain tension. Reset the rear wheel position by letting it settle home with the bike vertical on level ground. This ensures that the axle will be at the top front in the dropouts, then close the QR tight. BTW- if it's the hub, it'll creak sitting or standing.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.