If it were me, I would take all of the bolts out of the stem and grease the sides and shoulders of their heads, the full lengths of their shafts, the wedge itself (if a quill stem), and under the rim of the quill cap if so equipped. Basically, if it's not a weld, grease it all over.
A drop of TriFlow between the bar and the stem may silence a noise there, also, but make sure it isn't going to slip as a result.
If it's a threadless stem and headset, I'd do likewise with the entire stack of headset parts. Anything that touches something else has the potential to shift and make noise, including the spacers, the bearing cartridges (if so equipped), the stem on the steer tube, and even the cap where its rim touches the stem. I wouldn't get carried away; it only needs a slight film of grease to provide some lubricity.