Last summer my a/c started making this grinding noise when it would run for a few hours. After talking with some HVAC people, they suggested I use a less-dense filter in my furnace (I had been using 3M Filtrete). I switched filters and didn't have a problem any more (it was near the end of the summer, though). All winter, my furnace ran perfectly, no noise. I turned my air on for the first time a few days ago, after about 20-24 hours, it started making the grinding noise again. The noise is coming from the furnace itself. I turned off the a/c and the noise stopped 30-60 seconds later.
I changed my furnace filter (which had a few small clumps of dust on it) and am trying to run my a/c again today. Any thoughts/suggestions on how to fix this problem? The noise is rather loud.

You could be getting a squeeling noise from the unit pulling air past a small crack in the frame or door panal. Also your blower motor might be overheating or might have bearing issues. Might want to have an hvac tech come and check it out. If it is the motor and it stops working you won't have a noise but you won't have cold air either.

Also what kind of filter are you getting now? And which color of the 3m were you buying before?

I was using the red 3M Filtrete filters before. Now, I'm using a cheap one from Menards (don't remember the brand).
Thanks for your help!