Rattle at medium throttle

A couple months back I bought a 2013 Prius with 180k. It has a new head gasket, valve seals, water pump, thermostat, and the EGR cooler was cleaned 10k miles ago. It's been driving fine and I averaged 57mpg for my last tank of hilly highway driving across PA, which seems pretty ideal. In the last two weeks I started to notice a very faint rattle. It appears between 45-60 mph, only after the car is fully warmed up, and only at mid throttle. (specifically when the HSI is about 1/2 way through the ECO area) It instantly disappears when I change the throttle position, never appears when the HSI is in the power bar, and the engine sounds fine at idle.

I would describe the rattle as similar to top end tick or a timing chain guide sound, definitely not a knock. I can't always get it to happen. It is not the hood prop or the coolant return hose mount next to the passenger side headlight, but it originates in that area of the engine compartment.
The oil level is full, and I did an early oil change after I first heard this sound but it didn't solve it.

Any ideas or similar experience? It could be a preignition issue, but I'm not sure exactly what that sounds like or if it would be so intermittent. It's happened across three tanks of gas from different stations so bad gas is out. Thanks for any input you can provide!

 

I have receipts for parts for the work that the previous owner did, and I did the thermostat housing. No coolant loss or temp issues since the gasket replacement.
Replacing the PCV valve and adding an oil catch can have been on my list of things to do and I agree that wouldn't be a bad idea, but I find it hard to imagine it would clog up or fill with oil to the point of being a problem so soon and I wouldn't expect an intermittent rattle to be the symptom.
I should add that it doesn't use a significant amount of oil either, about 1/2 quart in a 4000 mile interval.