Originally Posted by
jadocs
What you are hearing is your chain. Think about size of each link on your chain. If it travels around a small/smaller cog it is going to be articulating more. On larger cogs it articulates much less as it travels around (that's why ceramic speed made those ridiculously expensive oversized pully wheels - it's supposed to increase drivetrain efficiency).
This (I came here to write the same answer -- it's most likely chain noise).
One other thing to check for, just in case: make sure the rear derailleur's jockey wheel isn't touching the cog. That can make a "brrrrrr" sound.
If it is touching, tighten the rear derailleur's B-tension screw until it no longer touches.