I hear this loud grinding noise coming from my back wheel...it goes away when i get to about 4500rpm and then it comes back... the car isn't driving funny its just really loud and annoying

 

Your rear wheels have nothing to do with engine RPM.

Is it at a specific speed?

Anyway, have you even attempted to actually look at it? That should've been the first thing you did...

 

havent had the time off... im gonna try to get it on a lift sometime this week

 

your calipers are probably seized or if you done the brake pads back there lately, then your rotors my be uneven.

 

When I first bought my max I had a very loud grinding noise coming from multiple wheels at various speeds. It took me a while, but I eventually figured out that it was the shrouds around the brake discs that were rubbing. I bent them out a little, and no more noise, but if you didn't used to have the noise and then you do all the sudden, then I would definitely think its a brake problem.

 

It could also be your lug nuts are loose. Trust me on this one haha, its a dumb mistake but I put on my lug nuts one day a long time ago and forgot to tighten them all the way because of a distraction (girls in bikinis doing car wash down street) and I forgot about them and drove for a while and then some lug nuts fell off without me knowing and all of a sudden I heard a loud grinding, frog ribbit-like noise, and when I looked everything over I saw I was missing two lug nuts. XD

 

+1 on the seized brake caliper, or it could be nothing more than you've worn the pads down and are now hitting the wear indicator. The metal wear indicator does make an awful grinding noise against a spinning rotor.

Pull the wheels and check the pads 1st.

 

I have the same issue with my right caliper. Sometimes the pedel gets realy firm. I know I need to replace but I've been lazy and it's been hot as a biotch in NC.

 

To add: Check if there is something behind the shroud and rotor. I had that happen to me once.

The other time, the shroud was bent and hitting the rotor, and the sound would go away at a higher speed since the rotor was cutting it away. I eventually bent it back out of the way.