Weird Noise

Got a strange creaking noise coming from somewhere but I can't quite
pinpoint exactly where. It only happens under a medium load with both hands
on the handlebar. It's in time with my pedal stroke (both sides) but if I
increase power or take one or both hands off the bars it vanishes
immediately.
At first I thought it was the adjustable stem but after a re-grease the
problem persists. It seems to be coming further forward than the bottom
bracket and I'll give it an overhaul next anyway (fingers crossed that kills
it) but I'm concerned it might be something more serious like a crack in the
frame or somewhere down inside the stem. There's nothing obvious I find from
a visual inspection and I can't *feel* the noise in my hands or feet to
pinpoint the source. I can only hear it (loud enough to hear above the ipod
and traffic).
Any suggestions?

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