I've got a brand new 2012 Fit and when I hold the revs at 1250-1500 I get a strange rattling noise and I was wondering if this is concerning. I have had Hondas in the past and I know these four cylinders can get noisy, not really able to hear it in the car except when I'm parked or next to a wall.

 

That sounds like a loose heat shield on the exhaust or a rattling catylitic converter. I wouldn't be too concerned.

 

I agree with the loose something or other.Exhaust shielding generally doesn't cause a knocking sound but in any case it needs to be traced.
Rattles are different and often are loose shields or brackets. Have someone assist you in tracing the sound. If you can use a metal pipe to hold against various parts of the engine with your ear at the outer end and try to locate the location of the knocking by the loudness. If you find it is in the engine a trip to the dealer is in order especially to document the 'problem' by your copy of the work order even when its warrantywork.

 

its a 2012 go to dealer let them check it should all be under warrenty? i think