Update: I don't think the issue is chain rubbing. I hopped on the bike this morning and rode it around my street a bit and heard the noise quite clearly coming from the cassette area. It definitely sounds like a grinding noise...very gritty sounding and I don't have to pedal too hard for it to make the noise. I can't imagine the frame flexes that much!

I'm taking it to the shop I bought the bike in about 1/2 hour. They've been great with their service and always very accommodating. I'll post another update after I find out what the problem is.