Clunking noise at front of engine when baring engine by hand

My 2010 Prius just turned over 206k/mi. I was getting misfire on cyl 2 code. It started to occasionally have the death rattle on startup if it sat for a few days. Other than that it was running fine, getting about 43mpg mixed driving. I was planning to do the EGR system & I/M cleaning and change sparkplugs soon. I left for work @ 4am in freezing weather, about 5min into drive I noticed the over temp light come on for about 10 seconds. Based on internet research I expected water pump was going out so quickly ordered a new one. However, when I looked under the hood, I noticed the coolant level was low. That was something new. The hose from reservoir that wraps around front of car and connects to small metal tube on larger metal pipe below EGR cooler had a pinhole leak in it with some signs of dried coolant residue on hose and on top of PSD so I assumed (hoped) that was the culprit. Ordered hose and started disassembly. Plugs looked pretty good, 2 from cylinders on PSD end had light brown crust on negative electrode but otherwise looked great, other 2 looked very clean. IM had typical oil pool in bottom, EGR ports were open but somewhat restricted. EGR cooler was somewhat restricted, but light still showed through clearly. Over-all not bad I thought. Guessing this service had been done before we bought car with 85k/mi on it. Since I had slightly elevated oil consumption (maybe 1qt / 5kmi) I set all 4 pistons @ mid-stroke & put MMO (Marvel Mystery Oil) and Chemtool B12 in cylinders to soak overnight, I also worked pistons up & down about 1/8 turn to work solvents into piston rings. I used an eductor (vacuum venturi) to suck all the liquid out of cylinders after I was done soaking them. Everything cleaned up nicely, new PCV valve, IM, EGR & water pump all went back together fine.
At this point I wanted to spray chainsaw gas mix oil into cylinders to pre-lube cylinder walls before putting (old) plugs in (for initial "dirty" start-up). While baring engine around to get pistons to BDC before spraying oil mist into cylinders I heard what seems an unusual noise. From above I thought it was coming from PSD area, so I removed inspection cover for flywheel to see if damper assembly might have broken springs laying inside. But when baring engine over from underneath car the sound seemed more like it was coming from front of engine, possibly timing chain cover? I would describe the noise as a "clunk".

YouTube Link to short video of flywheel to show speed and direction while I was baring engine back and forth:

Is this normal, or is something bad going on with timing chain? I could have forgotten, but don't remember this noise when baring engine around for piston soak process.

Also, after more reading on PriusChat I fear I might have a failing head gasket. Cooling system seems to hold pressure and vacuum, but AI says it is common for head gasket failure on Gen 3 Prius to only show up when engine is hot. I was planning to put it back together, warm up engine, then pull spark plugs to inspect cylinders for coolant seepage while system is hot and under pressure when this noise showed up. Now I'm a bit afraid to run engine.

 

Thank you both for the feed-back and really appreciated the excellent informative (if not a little depressing) videos @rjparker. I ordered a borescope and cooling system pressure test kit. Will let you know what I find there. I suspect the answer will not be good.

Still wondering about the odd noise when baring engine back and forth, is that normal? Maybe the timing chain? Maybe timing chain uses hydraulic tensioning that only keeps it tight when engine has oil pressure? Has anyone else noticed this sort of noise when baring engine over? I guess it was running before I tore it apart I could just ignore that and try running it again - guess I'm just a little paranoid.

The other thing I'm worried about now is down-time finishing off an older hybrid battery. That put the nail in the coffin of our 2005 Prius when we had icey weather for 2 weeks and car was left parked. When I went to drive it again the battery failed. Parking an older hybrid is almost the worst thing you can do as the battery needs regular "exercise". Battery was probably already compromised, but it was still running fine @ 350k/mi until I parked it. I am 1 week into this repair now and starting to get worried. I was hoping to at least get it running again long enough to bring up to temperature and top off the hybrid battery

Also, one video I watched by Car Care Nut about blown head gasket on a 2010 Prius it looked like he swapped out a relay in the fuse box for a special push button device to crank engine over for checking compression. Has anyone seen this before? Is that something a DIY person could get?

 

You rightly interpreted "baring". That was a term I saw used and seemed right...but guess it isn't . Maybe a non-American contributor used it?

Thank you again, your description of "releasing timing chain tensioner" sounds like what I could be hearing. I will proceed and assume that nothing is on the verge of self-destruction (well more than was already the case). I don't currently have access to Toyota Manuals and never had one of these apart so still learning what "normal" is.

Sounds like to run a compression check requires special procedures / techstream.