Black marks on white shirts. Help!!

[twinsufficient]

My kids' white school shirts keep getting black marks on, mainly on the collar tips, that won't come off. I know it's the washing machine that's causing this and have cleaned the seal and done a drum clean but it keeps happening. Anyone got any tips please!

Have you tried a drum clean with a couple of dishwasher tablets in?

Vanish stain remover. Leave it on I think for 30 minutes and then wash as usual.

Sodium percarbonate in with the wash should get rid do them. Re the cause, machine clean with vinegar? Why don’t think it’s the machine? That would leave random marks not specific areas consistently.

Is your washing machine starting to get really noisy on the spin cycles and/or making a thumping noise? That might be the bearings going and the black marks bearing grease.