Please tell me it might not be head gasket

Cleaning the EGR at this point won't do anything; if you already have cold start rattles you already have water.

If you want to convince yourself, park the car somewhere you can work on it and let it cool, a day before you have spare time -- pull the spark plug from cylinder #1 and use a cheap borescope to look inside.

> I don't know what test he did but he said it wasn't the head gasket

Not all shops are the same; some will insist that a combustion gas test is enough of a test to determine whether the HG is any good. What usually happens to the Gen3-ECU-managed 2ZR-FXE is that loss of sealing due to thermal cycling lead to coolant entering the cylinders after the engine cools off, and this creates accelerated wear through:
- the creation of preferred channels for water to flow
- the loss of coolant, generating more hot-spots (you've seen this with your overtemp event)
- the presence of incompressible fluid in the combustion chamber. Eventually, you'll get unlucky, and enough fluid will enter the combustion chamber after a cooling cycle to destroy a piston.

Think about it like this:

- You have a cold-start rattle (something other than air is making its way to the combustion chamber after the car cools overnight)
- You are losing coolant somehow
- You had a known overtemp event in an engine well-known for responding poorly when it overheats.

Getting it taken care of is cheaper than buying another car, but you probably need to find a shop that works on these engines, because (given the overtemp event) you probably have a slightly warped head they need to check.

 

OK; I'll bite - It's not the head gasket.
After the mechanic has bent-you-over for the small repairs that isn't going to solve your problems - he'll hit you with the head gasket repair or whole engine replacement.
1. few hundred for spark plugs
2. few hundred for ignition coils.
3. few hundred for fuel injector
4. few hundred to clean and/or replace the throttle body and MAF
5. maybe replace the fuel pump, water pump, O2 sensor.

After all that; they'll probably condemn the engine and tell you that you need a new one - because they're not going to take the chance that a simple head gasket replacement will fix it! After all the money you've already sunk into it.

NOW that's a profitable shop....... That was sarcasm; but those gen3 are notorious for blowing head gaskets. How many miles on her?

Seriously; take the car to an honest Prius specialty shop. A compression, leak-down and chemical test of the coolant for exhaust gases needs to be performed with the engine COLD. Hope doesn't change the outcome of the facts, but you can waste a lot of money in pursuit of a wishful outcome.

Good Luck.

FWIW; These cars don't store engine over-heat in the electronic logs. Your temperature lamp turns on around 248F; your normal engine operating temp. is 185F-210F max. - so that intermittent temp. lamp flashing is a BIG deal and your running your engine HOT; which causes blown head gaskets and warped engine parts. I don't know if the car buzzes when that lamp flashes, but it could be flashing a lot and you just haven't noticed it. If coolant levels are low, not touching the temperature sensor, your car will also be running HOT and the lamp will NEVER flash - sensor NOT touching the hot coolant. NONE of the above paragraph is logged by the ECU........

 

Find a mechanic who can do a proper video borescope inspection of the cylinders while the cooling system is pressurized with a test kit.

That's the quickest, cheapest, real test you can do to verify the status of your head gasket. They're either going to get video of pink coolant dribbling into one or more cylinders, or they aren't.

 

how many miles on her?

 

Engine needs to be cool to cold too. Early head gasket failures can seal itself up as engine warms up and the aluminum expands.

YMMV

 

You can have identical symptoms as head gasket failure and have it not be a head gasket failure if the misfire codes are on cylinder 3 & 4, rather than cylinder 1 & 2. If you're lucky enough for that to happen, it's usually easily repaired in less than a couple hours with new spark plugs and coils...

 

No other misfire causes loss of coolant. Most shops run a combustion gas test and it passes on these engines even with a confirmed hg leak. Your question should be a hg repair or replacement engine.