Engine Squeaky sound when let off the gas pedal

On my 2005 stang I can hear a disturbing squeaky sound whenever I release the gas pedal. Like an old door… It does not sound when the engine is off. And with a high speed it is also less clear but may be just due to the engine sound. It last for a year but as I do not drive it often I did not mind. Now with the season it annoys me like hell. Any tip or experience with such an issue? Should I lubricate sth under the hood? note: Aprox 40k miles only. Thanks!

Thanks. I will check that this week but as the throttle body is quite new (ca. 5 years ago changed to Ford Perform. racing) I am wondering if it might be anything else. Any other tip please?