I have a Craftsman 917.203810 that a customer brought in with a "deck problem". I discovered both spindles were worn out. I replaced both mandrels, blades and the belt. The idlers are both smooth and tight, so I left them alone. It still has a terrible vibration when the blades are engaged. Both mandrel pulley's spin true and smooth. I ruled out the spring tension by pulling it as tight as possible with a cord by hand while the blades were spinning. Is this a idler arm problem? There is no obvious damage to it, so I hate to just throw parts at it. Below is a video showing the belt slap that's causing the vibration.