In the last few weeks a creaking sound has developed somewhere in the stem/stem extender/ steerer tube assembly. I noticed it when going over bumps, but only when my hands are on the handlebar. Riding hands free there is no creak. I managed to recreate the sound while stopped by gripping the handlebar and frame and pulling up and down on the handlebar. I suspect it is coming from the stem extension (Delta brand). I loosened the stem extension bolts, twisted it a few times then retightened the bolts. The sound was reduced but not gone. I don't have a torque wrench yet so I was hesitant to reef on the bolts too much. I did notice some friction/squeaking while tightening the bolts and am wondering if they should be greased.
Take apart. Clean all contact surfaces (inner/outer) until perfectly clean. Wipe with alcohol.
Clean and regrease all bolts. Reassemble.
(Optionally) Stop by your local bike shop and ask for a torque wrench to tighten the bolts.
Repeat the steps above as needed. Expect it to start creaking again in a few months. There's a lot of force going thru the extender.