21HP, 46" model 917.289244 started to rattle loudly during a mowing session recently. The rattling stops when the brake is depressed. I can find no missing parts. The rattling returns as soon as the brake pedal is released

Hi,
I would say you either have a chunk out of the belt or a flat spot on the flat idler in the drive system. Please see attached for pictures from http://www.SearsPartsDirect.com and Managing your life just got easier | ManageMyLife using you furnished model number.
Peace,
Clarke

Thanks, Clarke. Turns out a small twig was wedged into the centerspan idler. Removed and back in business. Thanks again