Dreaded rattle at start up. But not everytime.

I have a 2014 Prius with 215,000 miles. I have the rattle, knocking, shaking at start up. Not every start up nor every cold start up. If the car is used the day before then it generally will rattle, etc. the next morning. If the car is not used then the next morning it starts smoothly. Occasionally it will rattle on a start up while using the car during the day. I have had to add some coolant. I did the test for combustion gas in the coolant with the engine combustion leak detector many times under various conditions and it always came back fine. I did the test to check the EGR by taking one bolt out and loosened the other and placing a putty knife at the EGR. It still rattled,etc. No check engine light. I checked for a code anyway but no codes. I took it to the Toyota dealership for them to fix. They diagnosed it and did a software update. $412 later still not fixed. At that time I didn't know that if the car sat without running it wouldn't do it. We don't use the car every day. I have changed the plugs and coils. Didn't fix it. Toyota dealership wants me to bring it back but I'm not inclined to do that. Does this seem to be a blown head gasket even though the combustion leak test didn't show it?