I have a threaded headset with a Deda NEOS quill stem, drop bars and clip on aerobars. Recently when I am riding in the drops, I have been hearing significant creaking coming from the point where the stem bolts around the handlebars. As far as can tell the bolts holding the handlebars in place are tight and I am afraid to tighten them further because I had a friend strip out an identical stem by overtightening it.

Should I be worried about this creaking, and if so, any suggestions as to what the problem may be and how to fix it?