Knock at start up

Hello all, my wife went to start our 12 Prius V this morning and it had the ever famous knock after start up but this time for a good 30 seconds. Normally this only happens in the winter for maybe a second or 2 but today was new. No check engine light. No coolant loss or oil issues. EGR cooler has been replaced as a preventative. Spark plugs were done at 120k and the car currently has almost 180k on it. So what is going on? Thanks!