Loud rattle when engine kicked on

Hello. Looking for some help diagnosing my problem before deciding which mechanic to go to (dealership or my general mechanic). I have a 2010 with about 215,000 miles. I started it this afternoon about an hour after driving it. Things were normal after I pushed the ignition button but about 15 seconds later when the engine kicked in, I had a very loud noise/rattle coming from under the hood. I shut it down and restarted it. Same thing. It went away after 5 or so seconds and drove fine. Not sure if it matters, but AC was on. Any help diagnosing is greatly appreciated. I'm going to take it in but not sure whether this needs to go to the dealership, which I typically like to avoid due to cost. Thanks!

 

Thanks, PriusCamper. The coolant level looks good. I didn't open it to listen to the sound since it looked fine. Should I still open it to listen or might that create a problem? I will start it up tomorrow with the AC off. I was able to start it up without a problem after I originally posted this and drove it for a fairly long trip with no issue.

Another issue that may be real and related or may be a figment of my imagination is that the last couple of days, when I start driving and accelerate, I've gotten the sensation a few times of some shaking- like I'm accelerating on a bunch of rocks, marbles or wet surface and need to accelerate more slowly (in reality, I was not accelerating quickly at all). Does that help solve the puzzle at all?

Bisco, when I first purchased it (~195,000 miles) was the one time I did have the dealership inspect it and do some work. This included replacing a timing chain tensioner gasket, transmission fluid flush, replacing sparks, clean & re-adapt throttle body & reset settings, fuel injection service, clean all fuel & air passages and cleaned MAF sensor & EGR circuit. I will say that the invoice doesn't explicitly say "cleaned EGR circuit" although I asked the service manager twice if it was done and he confirmed that it was so my long answer to a simple question is that I think it was cleaned about 20,000 miles ago.

Thank you both for your help and if you have any other ideas or advice, I'm all ears!

 

Thanks PriusCamper. I did open the coolant reservoir cap and did hear a sound. Does that pretty much seal the deal that it's a head gasket issue or are there other possibilities?

Thanks for the help nathenvan, tankyuong, & rjparker.

I may be getting ahead of myself, but how bad of an idea is using block sealer?

 

Given that I recently had an oil change (synthetic 0w20), the fact the fluid is not low, and the fact that the noise stops after the initial startup my mechanic suggested that the noise might be from the timing chain instead of engine. He suggested starting by changing the oil to 5w20 to see if the noise stops and then bring it in if it doesn't. I'm going to give that a shot. Does this make sense. Does the sound coming when I opened the coolant reservoir necessarily mean that it's the head gasket? The boroscope inspection is a good suggestion. Can you give me a sense of the cost? Thank you.

 

For anyone still watching the thread, I started it this morning and when the engine kicked in I had crazy vibrating near the engine. Also had significant vibrating/shaking when accelerating about 30% of my starts from being stopped. I got it to the mechanic who drove it with me. Of course it didn't do much for him, but I showed him a video (which is apparently too big to post here, b/c I tried) of the shaking when I started it up. He's going to observe it when he starts it up tomorrow morning. Thanks to all who have offered advice. I'm still all ears if anyone has anything to add. Frustrating because I just spend a bunch on suspension and tires.

 

When you guys say "too little, too late" are you saying that for the head gasket or for the entire engine?