I have a 2018 Toyota TRD with just over 17000 miles on it. Lately after the vehicle has been running I get a loud slapping sound at start up from under the hood. Sounds like a hose or wire harness is loose and is hitting something. Doesn’t sound like lifter noise. I found the big plastic piece that covers the engine was loose and a windshield washer hose was out of its retaining clip. Fixing them didn’t fix the noise. I also removed the flimsy front license plate bracket. That wasn’t it. So I’ve had the wife start the truck while I looked under the hood and it wouldn’t do it. But when I drive it to the store and start it, it will do it. Any of you Toyota guys ever deal with this issue? I’d appreciate any help.

Maybe something on the hood itself + gravity.

Our toyota Rav4 was doing this and it ended up be the timing chain and guides were rattling on start up especially on s warm start up.

2z. With so few miles I hope that is not the issue. What was done to fix it?

^^^^^^^^
Timing chain guide(s) have com loose/broke and timing chain is slapping against the timing chain case. That's what it sounds like from your description. I had it happen once in an old Tacoma. If that is what it is, if not fixed, the chain will quickly wear through the chain case and allow radiator water into the crank case. That water will almost immediately destroy the main and connecting rod bearings.

Pull the valve cover off and look down along the timing chain and see if the guides are in place.
If one is broke you will have to pull the oil pan off to retrieve the broken part as well as pull the timing chain covers off.

Of course there are probably a half dozen other things that could be causing the sound you are hearing.

An errant strand or 2 from the serpentine belt can simulate that sort of noise, as well as a bad bearing in the idler/tensioner.
But ..... not likely with so few miles.
?

I had a slapping noise on my 2018 it was from the free oil change guy did not putting all the bolts back in the skid cover (to thin to call it a plate)

Law dog. That’s about the time this began to happen. I’ll have them look at it.

Law Dog I had the same thing happen to me on my 2019 Tacoma only the guy managed to cross thread the bolts and what they screwed into so I drilled out the ruined threads and put new bolts in with nuts on top. He also didn’t rotate the tires as he said he did but I was able to prove he didn’t he is no longer employed there

The 2018's do have thin skid plates...Except for the Pro, now that's a real skid plate.

With only 17000 miles it ought to be warranty work. Take it the dealer

junk it sell for parts ............

Maybe the loose nut behind the wheel? Sorry couldn't help it.!
Howser

I replaced the chain, guides and gears.

I bet a 3 year or 36K mile warranty.?? If so I would stilltake it back to the dealership if not the skid plate.

Got my 06 in the shop now. My engine sound when starting the truck has turned out to be a Fly wheel.

I found the plastic rear covers on the back of the bumper were loose. I replaced both of the plastic push in retainers, thought I had it but later in the day I heard the noise again. So I thought that maybe the truck is driven so little the timing chain tensioners have not adjusted. So I decided to burn a little rubber and put some pressure on it. Force it to adjust. I haven’t heard any noise since. I know this sounds silly but I’ll drive the truck again tomorrow and see what happens. I do have a date with Toyota repair set and in two weeks just in case.

Started mine today heard a quick noise after it fired up not tinny but more flop like sound one and done kind of thing. Thinking back I heard it before but did not follow it enough to know if it’s a every time thing.

Well my last experiment didn’t work. Looks like it’s off to the dealer for a check up.