2011 Prius - Knocking

Your fear of the head gasket has just shown it's true form. You'll need to replace the head gasket or get yourself another engine. No other real solution

 

Did you use a bore scope to see the coolant leaking into the cylinders?
It doesn't take a lot for the engine to knock.....

Sounds like the head gasket is leaking... it will only get worst. The longer you wait the more it will
likely cost you....

 

Start saving your money to replace the heater core, radiator, coolant pump, thermostat,
small hoses, and engine.....

That was the WORSE thing you could have done. It's even worse than not replacing the head gasket.
It clogs up all the smaller passages. Extremely difficult to clean it out.

It's kind of like taking a pain pill, but not resolving the CAUSE of the pain.

 

A 50/50 mix of white vinegar and water “may” flush the stop-leak product.

 

Keep monitoring your coolant level - because it's somewhat likely that it's going to drop again.
It's also likely that the knocking that you heard was your little engine trying to compress coolant in one or more of it's cylinders.

Take a look at my avatar.

Trust me when I tell you that water does not compress very easily.
Metal bends when you try to do that.
So.....internal combustion engines have head gaskets, which in part act as a fuse to help prevent things like bent piston rods, or what aviators call an uncontained engine failure resulting in parts inside the engine punching a hole through the block.
Back in the old days - if there was a problem with the engine that resulted in a blown head gasket - the gasket could be replaced fairly easily since you could actually SEE most of the engine when you lifted the hood.

HOWEVER!!!
(comma!)
A blown head gasket isn't a 'problem'.......it's a symptom.
Just like a sudden low coolant level.
The "Bars Leaks" radiator treatment is intended to keep a small leak in your coolant loop from resulting in a low coolant level until you can find and fix the problem.
We will leave the wisdom of USING this product for another windy post!
Maintaining your coolant level is especially important since a high temperature event WILL wreck your aluminum engine in seconds rather than minutes - but that's ALSO another story.....

See the pattern?
Unexplained Low coolant is a symptom of a larger problem.
If your head gasket is leaking (and this is a very common issue with your car!) then THAT ALSO is a symptom of a larger issue. Contaminated engine oil is only one symptom of a compromised head gasket, and it does not happen every time.
A chocolate milk shake in your oil sump probably means a bad head gasket but uncontaminated oil does not always mean that your HG is OK!
There's only one definitive way to tell whether you have a bad head gasket which is to do a cylinder leak-down test.
You can do it for free of you have a parts shop that will loan you the tool, and you're able to remove the spark plugs, rotate the crank etc...etc.
The only problem is that this test is hard to do in a car whose engine is crammed sideways into a small car and covered with wires, hoses, plastics, etc.
Another slightly easier test that some people perform is to remove each plug and use a bore-scope - which thanks to the magic of Amazon you can buy for less than $30, delivered. If you know any conservatives in your area that will admit to owning a firearm - they probably have one that they can loan to you.
You do not need a very good one because IF you have a HG leak at least one of your four cylinders will be VERY OBVIOUSLY "steam cleaned" which is why you were getting the missfire codes.
In addition to being hard to compress, water doesn't burn very well either.
You might even find evidence of a liquid in the bore.
Read the ten longest posts in this forum that have EGR or Prius Death Rattle in the title and you might find the underlying cause to all of your problems or if you want to skip to the last chapter of the mystery novel look at @Mendel Leisk's signature.

Good Luck!