Long-time intermittent rattle/shaking suddenly became nonstop

Hey everybody, 2010 Prius, 201,000 miles, engine suddenly shaking/rattling like crazy. The first and most important detail of this problem is this: It's done this exact thing intermittently ever since we bought the car 100,000 miles ago.

Before, what it would do, is every once in a while (every couple months) we'd move the car in the driveway a few too many times without it fully warming up, and then on one of those startups it'd shake/rattle like crazy. Shut it off, restart, it might do it again for a few seconds but then clear out and run like it's fine.

Again, this car has done this, intermittently, once every couple months, a few seconds at a time then fine, for 100,000 miles.

A week ago it suddenly did it in a totally different situation with my wife at the wheel driving home from work, fully warmed up for 20 minutes, suddenly starts doing the same rattle/shake. She pulls over and calls me. I can't get it to go away.

Over that 100,000 miles, 7 years of ownership, the coolant has very slowly dropped in the reservoir from max to near low.

I don't think it's the head gasket. I've been a car guy my whole life, fixed all my own old cars for 30+ years including head gaskets, grenaded modified engines and total rebuilds.

Why not? Because I put it in neutral and the sound and vibration goes away completely, instantly. It now does this hot or cold, all the time, but only does it in these conditions: when it's in park, when it's in forward or reverse, and it's anywhere between idle and half throttle. AKA any time it's charging.

If I put it in neutral it goes away completely. If I drive it above half throttle it goes away completely.

It's giving me a code for a "random misfire"

At this point I'm thinking it's EGR or a horrible transmission/clutch spring thing.

I've cleaned the EGR system, intake included, twice in this car's life, once at 125,000 miles, then again about 5,000 miles ago. That's throughly blowing out and chemically cleaning out all the passages, the valve itself, the cooler, all that. I tried the EGR valve-to-intake-pipe blockoff trick thing and it didn't change whatsoever. No idea if that truly rules out an EGR system.

I pulled the ~2"x3" black plastic cover off the underside clutch part of the transmission facing the engine and felt around in there and I can't feel any detritus or broken springs. But there wasn't much room to feel around down there.

Did the plugs 15,000 miles ago with OEM factory replacements btw.

Ideas? Thanks

 

It appears the "it's been very gradually going down for 7 years and 100,000 miles and the car has driven fine basically the entire time" part has been taken as immaterial here. This is the original coolant put in the car from the factory 201,000 miles ago.

Also: this might not matter but it doesn't have any sign of a blown head gasket whatsoever and head gaskets don't just stop being blown the instant you put it in neutral or give it moderate throttle.

 

But the EGR has never been clogged in the entire time I've owned the car. The two times I cleaned it, I did it proactively and well ahead of time. I did the entire manifold and every single piece in between. The passages were all still flowing quite well at the time.

To make this clear again: This car has driven just fine with zero issues for all but a cumulative 30 seconds of its entire life in my care, those 30 seconds played out intermittently over the course of this car's 100,000-201,000 miles. Then suddenly it's doing the exact same symptom it's done very rarely since the very first week I've owned the car, except nonstop instead of for a few seconds.

And again, when I shift the car into neutral, the engine runs perfectly fine. This is not a head gasket, unless you're suggesting this head gasket can be turned on and off at will, instantly by shifting from park to neutral. It goes from shaking like crazy to running like brand new at the pull of a lever.

 

Are you ok? I haven't even suggested any of the things you're getting angry at me for doing and you don't appear to have read any of the post I put up.