2006 Toyota Sienna 78k miles rattle when throttle released

It appears I have stumped my mechanic. My '06 Toyota Sienna with 78k miles started making a rattle noise (my toddler says he hears a helicopter) every time I take my foot off the gas. It is most obvious between 25-60mph. The sound continues when the brake is depressed, but stops once the brake is fully engaged. My mechanic doesn’t know what is making the noise, but suspects something inside the transmission, and wants to replace the transmission (which is $5600 I’d rather avoid spending if possible). 78k miles is pretty young for a new transmission, especially on a Toyota, right? Any advice on where to look next?

Check the heat shields first, a tack weld might be needed.

My 04 Sienna did that a while ago and it was the heat shield on the piece just after the exhaust manifold on the bank of cylinders facing forward. The problem went away when it finally fell off. My 04 Sienna, with the same transmission as yours, has 200,000 miles on it and the transmission is original.

“My mechanic doesn’t know what is making the noise, but suspects something inside the transmission, and wants to replace the transmission” - OP

With respect… YOU NEED A NEW MECHANIC. Any mechanic that wants to replace the transmission for $5600 without any clue what’s causing the rattle should be thrown to the sharks… who would probably welcome him as one of their own.

Heat shields very commonly develop this condition. And the fix costs about $1.29 (for a large hose clamp) plus labor.
I don’t know if a loose heat shield is causing your rattle, but apparently neither does this “mechanic” (I use the term loosely). You need someone who will diagnose the cause of the noise before starting to replace parts… in this case beginning with the most expensive possible cause.

+1 to mountainbike’s comment.

The OP’s mechanic doesn’t know what is wrong with the vehicle, but he thinks that it is appropriate to go on a $5,600 fishing expedition? Yikes!

If I heard such a ridiculous statement coming from a mechanic, I would immediately look for somebody who is competent, rather than continuing to patronize somebody who wants to play guessing games with huge amounts of my money.

I should be clear: I actually love our mechanic and really respect the work they’ve done for us in the past. Dude’s got integrity, and is continuing to do research on the problem before pinning me with a new transmission and a huge bill. He used a stethoscope and pinpointed the noise to the transmission and is very sure the issue is in there, he just doesn’t have the tools to do a transmission rebuild. I’m going to take the car to a transmission shop to get a second opinion. I’ll ask about the heat shield. Right now we’re all doing as much research as we can to come up with an out-of-the-(transmission)-box solution.

“I’m going to take the car to a transmission shop to get a second opinion.”

That was going to be my next suggestion, but–if you value your wallet, and if you want an honest diagnosis–please be sure to NOT go to AAMCO, Lee Myles, Cottman, Mr. Transmission, or any other chain-run trans shop.

Look for an independently-operated transmission shop that has been in business for at least 3 years.

Maybe I has something to do with the traction control system, I think some those systems use deceleration sensors. Ask your shop to check that sensor to see if it is working, if you car uses one.

Ya know I tell my wife you go to a surgeon, they do surgery, go to a chiropractor they do manipulation, Now it may be the transmission shop says the trans is ok, maybe not, but I would start with a second opinion besides a transmission shop.

Yes, it is wise to be aware of the “If your only tool is a hammer, every problem looks like a nail” issue.