Mechanical gremlins

2010 Prius with 240K miles on it. Last winter it began "sneezing" on startup – the ICE was shaking and rattling violently on cold startup, sometimes throwing P0303. Cleaned intake manifold and EGR pipe (not much of a buildup, as I do it every 60K anyway), replaced plugs (old ones were fine though), checked coils… nope, still rattling. Then a few cans of B-12 magically fixed the issue.

Fast forward 4 months. Few days ago my car rattled on cold startup, then functioned normally throughout the day (I'm an Uber driver and put 100-400 miles on my car every night). Next day, it rattled again, and again normalized after a warm-up. Then I noticed that the engine coolant level is on "Low" mark, so I've added some (I usually fill to the seam).

…and that's when the real trouble began. Now it rattles on every ICE startup, not just the cold ones. For as long as ICE is engaged, like when driving on a highway, the engine sounds and functions pretty normal… but stop for a minute on a traffic light, and when it turns green – GRTTTTTT. In addition to that, there's a weird sloshing sound coming from the cooling system – on every engine startup. You can make the same sound with a mouthful of Listerine, IYKWYM.

Sometimes it throws P0303, sometimes it could go through dozens of rattling cycles without a code. Sometimes it throws P0303 and fires up a CEL, then functions just fine for a few start cycles once I clear the codes.

Checked: plugs (fine), coils (fine), throttle body (no pooling), oil (no milk today). Inspected 3rd cylinder with a snake cam (appears to be fine in both visible and UV). Tried: releasing pressure at the high point of the cooling system. It barfed bubbly coolant, which stopped sloshing for a while, but now it's back… Dammit.

A few questions:

• With a blown head gasket it would've rattled all the time, not just on startup… at least I think so. What else could it be?
• It appears that the engine cooling system is pressurized, every time I remove the cap (even on a cold car) it goes "pffff". I presume it's normal, as the cap itself seems to be designed with medium pressure in mind. However, it seems that there's some source of gases in the system. Sloshing is a good indicator, but also the fact that when I open the cap (even on a cold engine), coolant flows from the top hose. What the actual frack is going on here?!

In short – there seems to be no mixing of oil and coolant, but gases (likely exhaust) are probably getting into cooling system, and the 3rd cylinder is misfiring (or at least OBD thinks so) without any signs of blown head gasket.

Please pour balm of your wisdom on warts of my incompetence!