I have a 2014 Toyota Prius with a little over 300,000 miles that recently, whenever I start it up has an engine knocking noise for a few seconds then goes away and rides fine. I did some research and I changed the spark plugs and did an ATS fuel treatment but to no avail the knocking continues. I plugged my OBD reader into it and no codes are coming on, as well as no misfire readings. I began wondering if the EGR was clogged but was informed that it had been replaced at 200,000 miles. I took it to my local toyota dealer and they told me that the head gasket needed to be replaced and that it would be $4,000 with no explanation why.
Knocking is usual a (very bad) mechanical noise, but it could be a leaking exhaust manifold or other non-fatal problem. A blown head gasket doesn't make knocking noises. Spark plugs don't make knocking noises either, and neither do clogged fuel injectors or EGR valves.
At that mileage it is certainly possible the engine is just plain worn out. It might help if you can make a youtube video demonstrating the noise an post the link here.
