RPM dependent rattle

Okay, so at about 2500 rpm my engine makes a weird rattle. It's not very loud, and it doesn't seem to speed up as the engine speed increases (say, to 6000 rpm). However, if the engine speed is about 2000 or less you can't hear it at all. Any ideas on what it could be?

Performance hasn't slipped at all, nor has gas milage, but I have noticed that I'm losing small amounts of oil...burning or leaking, I don't know.

Thanks.

I notice that sometimes my dipstick rattles at certain RPM's and some oil forces past it when it happens. Check all the bolts on the valve cover and check the Oil Pan drain plug.

well... the rattle is pretty dependent on resonance... so the rattle may come in at a certain rpm which has a certain resonance..

my car does that...the heat shield rattled at about 2500 rpm... look at all flat metal parts... b/c they resonate easily... and if it was a loose bolt, etc... it would rattle all the time..
so just keep an ear out for it... and take your time... maybe listen in a tunnel, under a bridge, or get under the car when it rattles... it takes a while ,but you can find it...