2011 engine rattles HARD on startup after sitting a couple of days

My 2011 Prius has a very hard engine shake / rattle for seconds upon startup after it has sat unused for just a couple of days. When driven daily it starts smoothly and normally. It does seem to "chunk" harder than it used to when going from battery power to starting the engine to recharge (when sitting in Park). The car has about 140K miles. I did the EGR service several years ago, and I also recently did the brake booster / accumulator replacement very recently after that problem popped up. The car does burn some oil, but outside of that and this really aggressive shake / rattle on startup it is running great. Just worried that this is indicative of the next significant problem. Anyone have any idea what might be causing this or whether it is a concern? Alternatively, anyone have any soothing words of solace suggesting it's not a problem at all? Appreciate any insight.

 

This is suuuuuper bad news. Thanks to all for pointing this out, but man, what a bummer. I did the EGR cleanout myself at 117K and did the manifold as well, but didn't put in an oil catch can. I did the brake booster / actuator (DIY) about 1 month / 2000 miles ago (just did a road trip) at 146K, and the car has ~148.5K miles right now.

I just watched the first video on the head gasket and that job appears to be at a completely different level from the other two. Not sure I have the skills to pull that one off.

So if I do a head gasket, does the car last me another 100K problem free miles or what's the next big problem awaiting me? I feel like I've put quite a bit of time and money into this thing already as parts on that brake job were not cheap.

Thanks again to all for the quick diagnosis of the most likely scenario.

 

Just got through reading this thread:


Seems like the head gasket replacement is really expensive. Engine replacement w/ used Gen IV is a legitimate option (still very expensive), and my next major problem is probably the battery going out. Seems like it's going to cost a lot to keep this one running.

 

This is the first of three videos by Nutz about Bolts detailing a head gasket replacement:

 

So does anyone have an idea on what the deterioration time frame is on that head gasket? Like I said, it's only happening occasionally when I let it sit for a few days (assuming that's how long it takes for significant coolant to build up in the cylinders). Do I have a month? A year? Is there a way to slow down the process by avoiding some specific behavior (like letting the car sit)? And then is there a market for a Prius with a bad head gasket?

If I replace the engine does that resolve the EGR cooler issue as well?

 

Sorry - wasn't clear enough. I meant "Will a replacement gen 4 engine quit blowing so much oil into the EGR cooler that it ceases to become clogged and need to be removed and cleaned every 50K miles, thus eliminating many problems of the gen 3 car?"

After looking at the prices of newer priuses on Craigslist a gen 4 motor replacement is looking more appealing.