It's really hard to say what it is exactly. It could be anything. You'd probably want to have someone listen to it and try to locate it from outside the car while you rev it up in that 2500 to 3000 range.
I had a similar problem a couple of months ago and it ended up being the hood latch that had slid up causing the hood to vibrate at around 1500 to 2500 rpms. All I did to fix it was to loosen the mounting screws for the latch, lower the latch a little and then retighten the bolts. Note that the rpm could be different on yours since the weight and stiffness of the hood will affect the frequency range in which the hood will vibrate. A quick check to see if this is the problem is to have someone apply pressure on the hood while you rev. If this stops the noise... you've found the culprit. If not then keep looking. It could also be busted motor mounts, lack of clearance around the air intake tube or interference with any other aftermarket components.
good luck