weird problem

I'm having a problem with my 1999 Lincoln Navigator. When I get going and then stop at a light or somewhere, there is a loud humming noise coming from under the hood, and its pretty loud. If I give it some gas it will usually go away until I stop again. The thing is it only happens when it is cold out, under 45 degrees, it never happens when it is warmer than that. Sometimes the rpms will drop down a little before it does it, like down to 5000 rpms, usually idles around 6500 or 7000, and sometimes the rpms will fluctuate just a little, like 500 rpms when it is doing it. Anybody have any ideas? Thanks

sticky throttle...6000-7000 rpm isn't an idle. mabey the radiator fan turns on while stopped?

what kinda of vehical do you have?

woops. I meant 650 to 700, it goes down to 500 and it fluctuates like 50. Its a 99 Navigator. I don't think its the fan or anything, I mean its a loud hum that doesn't happen at any other times, and only when its cold.

is your a/c off

No, I usually have the air on but warmer.