noise underneath car when about 40 miles per hour

What is your question?

I recently bought a 2011 Toyota Rav4 for my granddaughter, I replaced the back drivers tire that had a bad belt. now there is a rumbling/grinding noise when you get to about 40 mph. I took it to my local garage and they said it was a defect of some kind from Toyota, but there wasn't a recall and they felt the car was safe. It's for my granddaughter, so I'm a little uncomfortable with that answer! The ride in general is a little rougher than I'd like, but it is 10 years old! I just want it to be SAFE!

What makes this problem better or worse?

when you get to about 40 mph

How long has your 2011 Toyota RAV4 had this problem?

just bought car last wek