Try to answer as many of these questions as you can and any other details you can:
(it would help if you could get a video recording of the noise)
How long does the noise occur, 1-2 seconds, 5-10 seconds, longer?
Does it sound like a very loud scary metallic grinding or more like a rattling?
Can you hear the noise if standing outside, hood open?
Do you see the oil pressure gauge (needle) move when the vehicle is started?
Where does the needle settle (about mid-scale or??)
Can you notice any difference in the oil pressure with a cold or hot engine ie: when the noise occurs?
How long does it take the oil pressure needle to move (one second, 5 seconds, longer?)?
What oil filter brand/model is installed?
Was the oi/oil filter changed recently, or has the oil filter been in use a long time ?? months)
What oil viscosity are you running?
What engine repairs or service has been done, in the past or recently before the noise started?
(any engine accessories replaced, starter, water pump, alternator, oil filter changed, belts replaced, ----)
Does the engine run normally otherwise?
The sound only lasts for as long as it takes to start the engine, so a couple of seconds maybe? As soon as the engine is running, the noise stops and seems to run fine. I don't hear the noise anymore while I'm driving. And again, I don't hear this noise when I'm starting it cold, after it's been sitting for 8 hours or more, I'm very routine, and typically only drive to work and back home. I'm only hearing this noise if I stop at the gas station on the way home and shut it off for a few minutes, then start it back up. Ever since Covid, I've been doing grocery pickup, so I don't even shut it off when I stop to pick up those orders. I mention that because I don't really have any other frame of reference for exactly how long it would have to be shut off to not make this noise. I just know that it hasn't done it after it has been sitting long enough to be completely cold, overnight , or while I'm at work.
It doesn't sound like grinding to me. The best I can describe is a loud metallic rattling sound, which is scary to me because it makes me worried that my engine is about to fail. This is not a small rattling sound, it's pretty loud. I will try to record it tomorrow with my phone.
I hear the noise while I'm inside the vehicle, starting it, so I'm sure I would be able to hear it if I was outside with the hood open, but I can't test that because I can't start it from outside the vehicle, and I don't have anyone to help me test anything, but like I said , it's pretty loud.
I will have to double check about the oil pressure gauge tomorrow. I'm not sure how long it takes for it to move, but it's always seemed to have normal oil pressure readings as far as I can remember.
I've been using the OEM Toyota oil filters for the 11 years that I've owned it, and I just changed the oil and filter about 3 or 4 months ago. I typically only drive less than 20 miles a day, just back and forth to work, so the oil isn't that old. It does leak some , but I always keep it checked and topped up. It's got almost 219k miles on it.
I've always used 10W30 oil, but I also add a quart of Lucas Oil Stabilizer when I change the oil. I've read good and bad opinions on the Lucas stuff, but I started adding it because I want the engine to last as long as possible, and they talk about it leaving a thin oil film on everything and "preventing dry starts", so it just seemed like a good idea.
As far as service or repairs go, a couple of years ago, I was having trouble with my battery not holding a charge, and it was a fairly new battery, so I figured maybe I needed a new alternator. I should've gone somewhere and had it tested, but I didn't. It looked pretty old, so I just decided to replace it. I did some reading on here, and ended up replacing the alternator, belts, idler pulley and fan clutch. ( I think I was also having a little squeaking noise at that time too). The alternator and idler pulley were OEM parts, I think the belts were Toyota too, but I can't remember what kind of fan clutch I bought. Those are the only engine related parts that have been replaced since I've owned it.
Yes, the engine seems to run normally after it is started. I haven't heard this noise any other time while driving.