Hi there! This is my first post on here after I�ve been surfing these forums for quite some time now. I have an 03 coupe, stock besides a muffler delete and plenum spacer. I�m advance I�m going to say I�m sorry for the longish post I�m just trying to give as much info as possible. And before I get people asking me if I�ve searched on these forums or on other forums or on google for the answer to this problem, I have and still have had no luck. It�s really pissing me off.

More or less a month ago, I�ve been hearing that god awful screech/whistling from my plenums alerting me of a vaccum leak. From other posts I�ve seen, I checked the main culprits. Hoses, gaskets, all that. I�ve changed the lower plenum gasket the other week and upper plenum gasket yesterday when I installed my plenum spacer, hoping that would fix the issue. You probably guessed, it didn�t work. Both PCV hoses were replaced with the Z1 silicone hoses a few months ago and I don�t see visible wear and/or tear. The whistle sounds like it�s coming from the front of the lower plenum where that lower plenum gasket is, but even with replacement it�s done nothing, that�s why I went to check PCV again. I will be looking at all other hoses one more time in a bit here so will update when I can.

Also, yes all bolts were torqued to spec and have been re-torqued multiple times to investigate this leak.

Here is a video of the noise I�m describing. You can clearly hear air from the front of lower plenum when I get close to it. At this point I�m just looking for ideas. Thank you in advance guys <3

https://youtube.com/shorts/p7a6or95DW0?feature=share

probably have to copy & paste to watch^ can�t figure out the hyperlinks unless it�s just not showing for me

I bought one of these for 130 bucks. It's a self contained unit with a built-in air compressor and you just hook it to your battery and it fills up the intake or whatever you checking for smoke. Any leaks should show themselves and I used it this morning to find out my intake gasket was messed up, valve cover gasket is leaking, and I had some pin holes in the PCV and throttle body coupler. I did build my own smoke machine a few years ago using a paint can, but this one is far superior and glad I purchased it.

You could also spray around the suspected area while the car is running with bottle of water, unlit propane, or carb/brake cleaner. If the car responds then your leak is in that area.

Ok thank you for the idea I will give the smoke machine a shot when I get a chance. I tried using brake cleaner and sprayed around everywhere I could around the intake and by some hoses and the car didn�t react a single time. Smoke machine will most likely be my best bet.

Sure you didn't pinch the oil fill plastic thing, it's SUPER easy to do and it makes that exact same screeching whistle sound.

Yeah I checked multiple times when I would remove the plenum and it�s not stuck in there. Still haven�t had time to figure out what�s good with it lately

At this point I would just take it apart, pretty sure you'll find something obvious.

man I�ve taken it apart a lot and haven�t found anything super obvious it sucks. Only other gasket problem it could be is probably the 2 gaskets underneath that lower manifold. Haven�t checked those yet but I replaced all those intake gaskets a few months ago then replaced lower and upper plenum again recently to see if that was the issue. My only options is those lower 2 and if it�s not them then I�m just lost lol. Gonna dive into it maybe tomorrow and see if they�re messed up or something. Maybe one blew who knows.

So my feeling ended up being correct. It was the lower intake manifold gaskets. I replaced those and oiled them up a bit so they would swell(read somewhere on here that it works), did the same with the replaced lower plenum gasket and problem solved! Thank u guys for the input u had.