[Solved]
Hey guys, I have an MCS 05. While running I hear a rattling sound coming from somewhere in the engine, I think maybe its from the passenger side but its really hard to tell.
I saw a few of things online that could be making that sound like chain tensioner, but I tested it and it isn't stuck at all. I checked the idle pulley and it isn't that either, doesn't seem like it's the alternator either. The sound only persists at low revs under 3000 rpm.

When I first start it, there is a very loud screeching it goes away pretty quick but it's really loud and scary and I don't think I should run it like this. The rattleing persists after this.

Any help is appreciated

Get one of these, if you don't have one, and poke around. It's definitely something that spins, so water pump ( on other side of engine), supercharger, belt tensioner pulley, idler pulley, ac compressor, alternator. My first thought was tensioner and idler pulley and belt. Just cuz it spins doesn't mean it's not bad. Good luck!

I ordered one and will try to identify the sound tomorrow. I was thinking running it without the belt to rule out all the other pulleys if sound still persists. But not sure if that could break something.

It should be fine as long as you don't do it too long.

I removed the Belt, and started it. Same exact screeching starting sound, and same running sound. If I rev it up to 3000 sound goes away. Maybe some metal on metal on the inside?

Does it go if you push the clutch down? Dual mass flywheels can cause a rhythmic knock.
could still be a weak hydraulic chain tensioner, when you rev the engine maybe its got enough pressure behind it?

Its an Automatic, but I do suppose it could be a transmission issue. I can take out the tensioner and check again, that sounds like a good step.

How’s the crank pulley? Maybe it’s the common failure of the rubber center and it’s rubbing on the timing chain cover. It could also be from the a problem with the timing chain, guides, or chain tensioner.

Chain Tensioner is good, I took it out and it retracts easily, but all that other stuff I would need to check. Is there a good video on checking the crank pulley? I don't know how to check that. As for the guides and timing chain they looked good when I changed the head gasket but I can certainly take them out and check again if nothing on the outside fixes it.

Pulled the crank pulley and that’s not it. Same sound on start up. So that’s not it. Running out of things to check that I can still crank the car to listen for the sound. I’m going to try the belt tensioner next I just saw that in an above comment.

I took off the hydraulic dampener on the tensioner and changed it out. But I don’t think that’s it. Sound still present on start. I figured I could at least check cam shaft for anything and took off the valve cover and found something interesting but probably not the actual problem.

1 rocker is missing it’s plastic holder and bevel. The other one was broken but still on there. So I am wondering if that little metal bevel part went into something, where would it be? Would it be in the oil sump? Hopefully it’s in there. I have more rockers so replacement isn’t a problem. But I wonder if that has anything to do with the sounds.

Interesting 🤔
https://www.waymotorworks.com/oil-pump-r50-r52-r53.html
I'm wondering if it's the oil pump?? You've eliminated all the parts outside the motor. Way says it's not a common failure but maybe it's your lucky day. Buy a lottery ticket if it is the oil pump🤣.

how can I check if it is in fact the Oil pump, I don’t see a video anywhere of anybody checking that. I tried listening for the sound with a scope I got and I can’t lock onto it.

Scope is best way so... I am not sure. Did you put the scope on the water pump side?

I had checked that side, but maybe I didn't quite touch the right spot. What is on that side that could make that starting sound? Transmission?

I wanted to update this thread in case anybody else ran into the same issue. I solved it after a while of not thinking about it. But of course after I figured it out, it felt like it was right in my face the whole time.

So first, the tapping sound from Video 1. That was the rocker with missing bevel in one of the images I posted, that was pretty easy to fix just changed the rocker out. The rhythmic tapping sound was the clue there and should have been obvious, but I only realized it was that after I put it all back together with the rocker fixed and the tapping was gone.

Second is the screeching sound. It was the starter, the pinion wasn't retracting all the way back so the fly wheel was grinding on the side of it and eating away at the teeth while making that devil sound. Should have been obvious since it only happened when I started the car and went away when right after. Removing the starter on these things is a PITA btw, you need a crazy long extension to reach it especially if it has never been touched before and the bolt is rusty.

So there you go if you have any questions let me know.

THANK YOU - @FuYA.
No one likes losing their mind over these anomalies.
Thanks for returning to this thread with an update and your resolution!