LS400 1UZ made a knocking sound after sitting

The noise did go away but was wondering if anyone has any insight. Car is a 2000 (2nd gen) with 380k miles.

This week the car had been parked for 6 days while I waited for a new fan bracket+bearing assembly to arrive. Install was tedious but not too bad.

When first starting it made this awful knocking noise. I switch it off in a second to remove covers and check for anything left unplugged. All good.

The second time it's a little quieter, this time I check for codes (none) and recorded the video at the bottom. The sound seems to be coming from the driver side front of the engine.

I start it for the third time to pinpoint the location with a socket extension to my ear and it's even quieter. I can't locate the noise, that method has never worked for me.

I decide to drive a little and not quite around the block and it's as quiet as it was a week before. Quieter actually, the fan bracket must've been on its way out for while. Was also smooth on a quick run up the highway.

I'm guessing after being parked for almost a week that oil drained out of something, but I think the oil system would pressurize in seconds and not the minutes it took to attempt diagnosis.

I have a factory manual but don't see an oil flow diagram.

Don't know if it makes a difference but the battery was disconnected while I waited for the fan bracket. Since it's my daily it has never sat this long since buying it in 2020, maybe 3 days at most.

Anyone know these engines well enough?

Sounds like a fuel injector or other solenoid to me. My ls400 sometimes had noisy fuel injectors. Could you hear it clearly with the hood closed from inside the cabin? 

Yep. It was audible from the inside when I started the test drive.

I hadn't thought of it being an injector, didn't think they get that loud.

I will admit I'm not familiar with these engines, but one week shouldn't cause any issues. 

Last week I started up my '23 BRZ for the first time in almost 6 weeks because of surgery/recovery, no noise. Other than it's usual DI/tractor sounding self. 

I'm with d on an injector. As long as the oil level is good and you don't think it's rod knock, I suspect it's find. And you're doing like me and letting the paranoia of something bad get in your head. 

That's a very timely thread as my LX470 has recently started making the same noise. The only difference being that while it quietens down, you can still hear it at idle and slighly above idle when the engine is hot.