I had that problem, it only occurred under high power and I couldn't replicate it off the bike either, I took it to a bike shop and it turned out to be the bearings in the freehub. Strangely it sounded for all the world like it was coming from the front, so I was worried it was a cracked fork or stem initially, and never thought of checking the back - apparently this is caused by the sound reverberating throughout the (alu) frame though.