I have a 2009 Civic SI and when ever i start driving i hear a grind at 18 MPH. It has about 1,200 Miles on it and has done it since i got it with 7 miles on it.
It does not matter what gear i am in.
It does not matter what RPM i am at.
It does not matter if i am on the gas or not. I can be coasting and once it hits 18 MPH i hear a grind, it is quite noticeable.
It only does the first time it hits 18 MPH then it does not do it again until the next time i drive it.

Take it back to the dealer?

 

That sound is most likely your ABS pump. Once you go over 18mph ABS turns on and does a self check. It shouldn't be a grinding, more like a buzzing.

 

Definately take it back to the dealer...i would have taken it to them the first time i noticed it at 7 miles....it's their issue and they need to fix it.

 

yeah, why would you wait 1200 miles to go to the dealer? I wonder about people...