Front End Rattle — NOT Undercarriage Shield

My 3rd Gen 2012 Prius makes a front end rattle noise. It consistently happens:

• At 65+ mph; sounds like something flapping and bouncing under the hood
• Upon accelerating from my first full stop after a trip on the highway; sounds like a vibrating gurgle and rattle
  • Listen to the sound here:

I've brought it in for service about this issue twice, but the mechanics haven't had luck identifying the source of the noise. They did try cleaning and reattaching the undercarriage shield and making sure that the hood insulation and plastic engine cover were secure, but the noise persisted. They blindly guessed that it could have something to do with the transmission damper assembly and spring and suggested a pricey repair with no guarantee that it would solve the problem.

What could this noise be? Any help would be much appreciated!

The vehicle is in stellar condition otherwise with ~48,000 miles on it and with no major repairs.

 

Thanks so much for your thorough reply!

1. The 65+ mph sound comes from the same area and is similar to the timbre/quality of the sound at the acceleration after the first stop, so I'm sure they're the related.
2. It seems to be coming from close behind the dashboard, behind the steering wheel area, and within the hood area.
3. The work related to the noise that I actually had performed was having the plastic undercarriage shield removed, cleaned, and securely reattached. The sound appeared pretty much all of a sudden on a long trip on the highway, not gradually.
4. The plastic trim, undercarriage covering (and oil change access door) are secure and don't rattle. There are not any cracks in my front bumper/spoiler or loose spots where a fastener/clip is missing. I'm careful around potholes and parking lot bumps. My wheel covers are tightly in place. I don't feel any vibration on the steering wheel or through the seat. These are all things that I have personally checked and have had the mechanics double check.
5. Yes, the mechanics assured me that they looked at the suspension, axles, breaks, tires, and checked the transmission fluid. That's a good idea about strategically placing microphones but I'm not sure exactly what products exist that would stay attached at such a high speed with constant wind and hold up to the heat in the hood. Have you had luck with placing microphones inside a hood? To clarify, the sound that I linked in my original post is very representative.

 

Yes, it's the noise at the ~4-5 second mark. That's the sound that happens immediately when I begin accelerating from a full stop.

Thanks! The vehicle is in stellar condition otherwise with ~48,000 miles on it and with no major repairs.

 

It sounds more mechanical to me as well. It happens pretty consistently at 65+ mph, especially if I'm accelerating to pass someone, and it happens almost every time as I accelerate from my first full stop after getting off the highway.

 

I sure hope so! I can't be the only one who's had this issue.

Yes, I'm all up to date with no outstanding recalls or anything to address.