Civic overheating.

[quote:0480735576="c0ba1t"]Recently my 2000 Civic (200,000 miles) has been running hot when the engine is idle. It runs at normal temps when moving AND also runs at normal temps while parked if I rev up the RPMs a little bit. Once hot, I can watch the temperature drop almost instantly when I rev up the engine a thousand RPMs or so.
I've also noticed a noise from the engine compartment lately, although i suspect it may not be related to the overheating. It sounds like rattling from a heatshield or bad bearings from something. It doesn't sound like it's coming from the water pump, although it's hard to tell.[/quote:0480735576]

Try replacing the thermostat-not it do the following below:

Have it block and pressure tested-do the block first to pinpoint a combustion leaking into the cooling system or a gas analyzer to sniff for hydrocarbons at the radiator fill neck.

Pressure test: do not do a pressure test if there's leakage at the headgasket this might cause coolant into the cylinders and lock it up or bend a connecting rod if cranked thereafter. The headgasket should be repaired before doing the pressure test.

If the block and pressure test passes check the following: ,Pressure test the Rad. cap,Clogged radiator,air lock in the cooling system ,Fan clutch,Radiator electrical fan,collaspe hoses,water pump.