Weird rattling sound when starting

IF miles are 150 to 180 most likely headgasket is blown
or could be dirty EGR cooler
or could be cloged egr intake ports
fastest way to confirm headgasket is to check you coolant if it is low your engine is likely done.
cost is about 1600 to 2800 at a shop. for that price I would just replace the engine with a gen 4

 

It's called the Engine Rattle Cold Start issue.
There are many videos and many post on how people try to make it better but no exact fix.
I would start with the EGR system cleaning first, like mentioned above.

 

Because of the way the Prius powertrain works, engine misfires from any cause will result in horrible-sounding rattles. The reason there has been no one exact fix found is that there is no one exact reason engines misfire.

Another thread just today demonstrated that one reason for an engine misfire can be that somebody did recent work on the intake and forgot to put two bolts back in. Probably not going to be the explanation in most other cars, but it goes to show the variety of things that can cause misfires, and the importance of not jumping to narrow conclusions too quickly.

One constant, though, is that misfiring isn't good for the engine, catalytic converter, or powertrain, and so it is worth making a good effort to find the cause before letting it go on too long.

Here's a classic old article where somebody let a pretty good car go for scrap price after overlooking a very inexpensive reason for misfiring.

Starting on page 15 of this Toyota Tech issue, there are a bunch of tips given on how to reproduce and isolate the engine conditions where misfiring is happening.

 

How many miles on it? Ever cleaned the Exhaust Gas Recirculation system, or Intake Manifold? See first link in my signature*.

* If viewing on phone, turn to landscape orientation to view sig's.

 

Good luck with it. Again: what's the miles on it? If it's over 100K you're getting into EGR clog territory. If over 150K it's pretty much a certainty you've got advanced clogging.

 

Does it always occur on start up? I have a similar sound if I shut down improperly, I mean if I shut down while the ice charging from 40% to 50%, if that is the case, wait until the ice stops before shutting down and the rattle won't replicate the next start up.

 

I've experienced that, suspect it's "another of the many roads that leads to Rome". Maybe that short-cycliing of the the engine does the same as a failing head gasket: leaves a little (uncompressible) liquid in a cylinder.

 

If you do venture into EGR system cleaning, it would be very helpful if you could contribute both the before-cleaning and after-cleaning flow test numbers, and your mileage, over on the thread we have for that purpose.

Many diagnostic scan tools and phone apps are able to pull up those numbers; you don't need something specialized and it isn't hard. But it does take a bit of forethought; obviously, you won't be able to post the before-cleaning value if it is an afterthought when you've already cleaned.

For the time being, rules of thumb like "over 150K it's pretty much a certainty you've got advanced clogging" are kind of because-Mendel-says-so statistics, but the more actual data we can collect, the sooner we will have good quantified ones to serve that purpose.