What could be causing the grinding noise in my washing machine?
You don't have a lot of info there.  But it sounds like your bolt that is holding your main bearing for the main drum is coming loose.  If it is a front load this is really easy to get to.  Take off the back and you should see a large bearing.  I had to replace mine for the same reason.  It is held by one screw and has teeth that make the main drum spin.  If the teeth are still in ok condition - which it just depends on how much you used it with that noise - then you just need to put the bolt all the way back in with blue loctite.  If it isn't glued in it will works its way out quickly.