There's been an increase in the 3rd gen Prius coming in with a knock in the engines on cold starts and even constant as the vehicle cycles from electric to gas. The fix for that is preventative maintenance on the EGR system before it clogs up completely leading to a catastrophic head gasket failure.
The EGR system maintenance should be completed ideally at 100k miles initially and then every 75k thereafter to keep the vehicle on the road in the most economical manner. A head gasket replacement can be costly depending on where it is done, some costing as much as $3k and as low as $2.5k, I am cheap and prefer preventative maintenance over the head gasket. When the EGR system is serviced, if caught early the EGR cooler can be cleaned but if left to become completely clogged then you will never get it to flow gasses as optimally as it once did and a replacement is recommended. In summary, if we keep up with this crucial system before it is completely clogged and gunked up, these vehicles will stay on the road longer and run better, while costing less to maintain.