I've had a faulty hydraulic pump throw sand like particles into the hyd system- learned this one the hard way- same cyl was repacked 4 times within a year (telescoping hoist cyl in a C500 clark)- after the 3rd time (hyd vendor would no longer warranty) apart we carefully inspected each seal, gland nut- etc & found metalic "sand"at each point- checked & found "sand" at bottom of hyd tank & covering filter media.

Truck had no hyd symptoms other than hoist cyl leak- removed hyd pump & found bearing surfaces rough (chrome was gone) & gears pitted. It's a long shot but worth a look if no other cause can be found.