Mysterious Prius Rattle

sounds like the egr circuit to me, wouldn't hurt to take a peak

 

Might sound like a dumb idea, but every so often my hood prop rod comes unhooked and rests on the engine and creates a rattle sound. If you haven’t been under the hood since the noise started you might check it.

 

Clean the the full EGR (Exhaust Gas Recirculation) circuit (cooler, valve and pipe, running from exhaust to intake manifold), and the intake manifold. Clean the throttle body and PCV valve (or replace).

By 165k yours is likely well along in carbon clogging.

Toughest, both to access and to clean, is the cooler.

Look into oil catch cans too.

I can post more info/links in a bit.

Addendum, here we go:

EGR:

The simplest way to see where you're at, is to check the degree of carbon build up in the EGR pipe, a stainless steel connecting pipe between the EGR valve and intake manifold. Watch @NutzAboutBolts video #16 here:

Nutz About Bolts Prius Maintenance Videos | PriusChat

Two or three other videos linked there too, for the full cleaning of the intake manifold, full EGR clean, and Oil Catch Can install.

Good thread:

EGR & Intake Manifold Clean Results | PriusChat

Another:

Oil Catch Can, Eliminate that knock! | PriusChat

Some tools worth having:

E8 Torx socket (mandatory)
E6 Torx socket (optional, but good to have, to remove the throttle body studs from intake manifold)
3/8" ratchet wrench, regular and long handle, flex head, you can never have enough (or 1/2 plus reducer)
1/4" ratchet wrench, or 3/8" to 1/4" reducer
Ratchet extensions: you can never have enough
Long needle nose piers, straight and bent tip
Ratcheting 12mm box wrench (optional, but makes disconnection of the EGR cooler from exhaust easier)
Torque wrench (3/8" and 1/4" both good to have)
Floor jack and safety stands (or ramps): basically some method to raise front, if you need to take underpanel off, which you may need to, both for access and to recover dropped items.

Comment regarding clamping of coolant hose, mentioned and or shown in videos:

1. When removing the intake manifold for cleaning, you do need to lift the throttle body off the intake manifold. Still, the coolant hoses connected to the throttle body have ample slack, enough that you can leave them connected, and just tie the throttle body to something adjacent, say the inverter wiring harness.

2. When removing the EGR cooler, removing coolant hoses is necessary, but if you drain 2 liters/quarts from the radiator drain spigot prior (into a clean container), the coolant level in the system will be dropped below the EGR components, and you won't spill anything. Just be sure to not tip the cooler when lifting it off (and catch the rear gasket): there are a few tablespoons of coolant trapped at the lower back corner.

Pour that into your previous drained coolant, and when done pour it back into the reservoir. If you've got the coolant bleed bolt (2010, 2011 model years), leave it open while pouring the coolant back in, till coolant starts coming out. For later model years, leave the topmost coolant hose on EGR disconnected till coolant starts coming out. Also, might help to burp the main radiator hose as you pour the coolant back in.