2010 shaking in reverse

May have got in over my head this weekend! 2010 Prius with 160k miles.

Following NutzAboutBolts videos, I did:

-drain/fill engine and inverter coolant, drain/fill transmission fluid, cleaned intake manifold/replaced gasket,changed PCV valve,cleaned EGR valve/piping/cooler, replaced spark plugs, and installed an oil catch can

Started it up, and it seems to run fine in park. When I shift to reverse, the car shakes every ~2 seconds and I hear a mechanical-sounding switching from in front of driver seat. Besides back and forth in driveway, I haven't driven it for fear of damage.

Plug gaps/torque are good; coolant levels good. Check engine light not on.

I know I've introduced a ton of variables here…any ideas what to check next?

 

Thanks JC. Will verify #1. Is the idea on #2 to let the fluids fully circulate before driving?

 

Thanks jerry! Now that you mention it, I did have the car on jackstands near front wheels while filling the transmission fluid. I fed a hose from above so I jacked it to be able to insert the hose into the fill. But I forgot to bring it back down for filling! The fill hole is near the back, so I think I probably underfilled it.

 

Thank you Mendel-I did use Toyota ATF WS.

I've got a floor jack,ramps, and 2 jackstands so I'm not sure how to get it raised and level safely. Any suggestions?

If I can't get it raised AND level, perhaps I can fill while it's on the ground, then jack up the passenger side to give me room to get plug in? I don't think any fluid will come out if I jack on that side but not sure

 

Made sure transmission fluid filled while flat. Car ran for about 30 seconds no problem. At ~6s into the video you can hear the rattling, this time in park. I've only run the car for less than a minute for fear of any damage. It doesn't feel safe to let this run like this for 10min but wanted to share in case this sparks any ideas.

Sounds at the end of the video are mostly from the car but some from me getting in to turn it off.

 

Fair enough! First time posting and I can't figure out how/if thread titles can be changed so I've asked the moderator to.

I've posted this info over in a FB group:

car makes rough rattling/knocking sound upon startup now, not just in reverse; engine shaking. Only code I am getting is P3191. Haven't run for more than 1min at a time due to fear of damage.

Things I've checked so far:
-plug gaps/torque good
-coolants and transmission levels good
-disconnected oil catch can and reinstalled PCV to IM hose
-cleaned MAP and MAF sensors with MAF cleaner
-EGR valve plunger moves freely (I did clean upon initial disassembly but perhaps other side still has gunk and not able to close completely)

I'm in the process of disassembly again to change throttle body gasket. Upon doing so I noticed I had not put the hose from the throttle body to the air filter box on well, so I may have had a leak. So I'll give it a go after reassembly and update.

I appreciate all the inputs!

 

UPDATE: Up and running! After tearing it down, there were a few potential culprits:
-the bellow to the throttle body was not on all the way, and seated somewhat crooked
-one of the bolts connecting EGR piping to valve was not tightened all the way
-I replaced the throttle and EGR to IM gaskets. I had replaced the IM gasket itself originally

Ran it for 10min in maintenance mode; during this 10min it had a few moments of rough idling (assuming this is clearing out any vapors from cleaners used and coolant filling in pockets?)

Just got back from a few mile drive (in regular mode). The only problem I'm noticing now is that it seems to hesitate and maybe run a little rough at around 15mph when accelerating. No codes being thrown.

Thank you all for the guidance and if you've got any ideas on the current issue I'd be grateful!