I recently purchased a used 550EX flash off of ebay. The flash works just great except for one thing. It is stinking loud. I believe it is the capacitor making the high pitch noise when the flash is recharging. I have heard other 550ex and they aren't nearly as loud as mine. Any advice on fixing this?

It should only be "loud" if it is shooting full power. Try half 1/2 , or 1/4 stops if you are controlling the flash manually.

-tmiller
Tim Miller Photography
http://tmillerphoto.com