Engine Rattle when sitting in heavy traffic

How long have you had it, and how much of that would be EV mode? A regular 3rd gen hybrid Prius would be well along with EGR clogging by those miles.

 

probably just the nature of how the egr valve operates

 

Try again: do you plug it in often, or just use it like a regular (hybrid-only) Prius?

 

Just trying to get a sense of the regular, non-electric miles on it. If it's done most of it's 170K miles like a regular hybrid Prius, you better get on EGR (Exhaust Gas Recirulation) and intake manifold cleaning. More info in first link in my signature. Opportune time to do it would be 100K, then every 50K thereafter, IMHO.

If it's left long enough head gasket failure is happening, reported here a lot with 3rd gen's. How's the coolant level in the engine coolant reservoir, failrly stable?

 

no, it isn't bad not to plug it in, it just behaves like a regular prius, and thelithium battery prefers to be discharged anyway.
the point is, until you have cleaned the entire egr circuit, you can't eliminate it as a possibility, and it needs to be done anyway.

 

Have you checked underneath for a loose catalytic converter shield? They will sometimes rattle if they get loose. Usually most noticeable when sitting in traffic, not/barely moving.