That noise was really bothering me so I decided to re-check all the belts. I took the timing belt cover off and found that the tension roller pin was not all the way in the block, causing the roller to run a few degrees sideways. It looks like I forgot to check if the idler pulley was flush with the block when I had the timing belt off. I loosened the idler nut, tapped the pin back into the block, the re-torqued the nut. The idler is now running true, but the idler bracket is a little bent, the pin pulled back out a little when I torqued the nut. I'll flatten the bracket next time I have everything apart to change the timing belt. I re-installed everything and fired up the engine, no more noise! The crooked idler probably didn't allow enough tension on the belt during adjustment, so it was probably slapping around, resonating at that certain speed.