[Yarp]

it’s a 2006 Ford Mustang. V6

when idling sometimes a rattling sound comes from what sounds like the front of the car.

it’s very noticeable, but very intermittent. Doesn’t happen every time and comes and goes when idling.

it is tied to the engine, when you rev the engine it speeds up before fading away.

I’ve got a video of it, I hope this helps.
Rattling noise video

 

[Imatk]

I noticed from the video you have a check engine light on. That would be my first step, plug in a code reader and see what CEL codes you get then go from there.

Difficult to say what that noise is from inside the car, would probably be good to have an "underhood" video, but it definitely doesn't sound good.

 

[Cobrajet67]

Like mentioned the audio isn't the best from inside the car. It could be something simple and minor like an exhaust rattle or something expensive like timing parts.
Since your CEL is lit you should have it scanned immediately.
Unfortunately, the 2005-10 Mustangs had timing parts issues whether you own the v6 or GT. The GT generally has a worse reputation than the v6 does at least until 2009 when Ford revised multiple parts in the 4.6 3v timing system. However, the 4.0 v6 that you have has issues with the timing cassettes thru 2005-10 especially at high mileage. There are two in the front and one in the back. The front cassettes aren't terrible to replace but the rear one requires removing the engine to get at it so it's not cheap or easy. My guess is that's your problem but the trouble codes will let you know the basic problem or at least give you an idea of what it might be.