First post here, first off thanks in advance for reading, and that's pretty awesome that the thread title field also functions as a search. 2010 Prius here at 120k miles. Bought it at 103k in Feb 2020. Spent almost a year not driving due to 2020, now am getting close to my usual mileage load of 24-30k per year expected. Once during the winter this past year, I got the startup knock. But it went away quickly, so I shrugged my shoulders like the mostly car-illiterate person that I am, with the exception of watching Scotty Kilmer sometimes. It happened one other time, about a week ago, but again went away quick. Over the last few months I've noticed that about midway up the PWR range (wish there was a tachometer) up through the top of the meter, I get a rattle. Not a chunky, car-shaking one like the startup. It's higher pitched, and I cannot feel anything wrong, just the sound. I didn't think too much about this because my brakes had been making horrible grinding sounds while moving with the wheel turned. Had been doing this since shortly after I bought the car. I figured that I was hearing more symptoms of that problem. Mechanic said the brakes worked fine, but would make noise until replaced. Finally had enough, and did a full brake job. The rattle is still there after the brake job, always at about half way up the red part of the gauge. It will do this about 70% of the time, other 30% engine sounds normal. Please tell me it's not the head gasket . . . . . . I have a video with the sound in question. 0:57 is where you can hear it the best.
Update: So this has been going on for the past 10k miles or so, just to clarify the timeline. I drove about a thousand miles this weekend (no exaggeration) and found that I can almost always stop this sound from occurring if I take it very easy on the gas pedal and accelerate slowly. Engine sounds perfectly normal when I do this, all the way up to the top of its RPMs. But if I need to accelerate quickly or need more power from the engine to go up a hill after a stop sign, that's when this noise will come back. Fuel efficiency is very good at the moment, pulling about 54 MPG. When I changed the brakes I also had them do the oil, and they did not mention any coolant present.
Oh and one other thing! The noise does not speed up as you depress the gas pedal and rev higher. It sounds like something shaking, and the best I can describe it (it doesn't come through perfectly in the video) is like if you were to take a box grater and scrape your fingernails against it.
Probably it's because of youtube's compression, but I'm not hearing the engine clearly on the video even with headphones.
Since it's 11 years old, have you looked at the motor mounts? If one is lose or broken it can definitely make strange rattling noises only heard at certain times.
I had a vibration at the middle of the power range (1500 rpm) that went away after the egr system clean, I could not hear a rattle but it is likely because I am deaf from working around planes all my life but detonation from a plugged up egr system would increase combustion temperatures and possibly create a rattle, vibration and also blow the head gasket... sounds like you have 2 of the 3
Another easy thing to check that often rattle in cars of this age would be exhaust system heat shields, some of their mounting points tend to rust and brake often creating a metallic rattle at their resonant frequency, checking for this type of rattle is easily done by jacking up the car and tapping the heat shields and other exhaust system components to see if you can replicate the sound
