Diagnosing an intermittent squeal on 996

Hey everyone, looking for a little help diagnosing a squeal on a 2004 996 C4S. The squeal just recently showed up and happens only under certain engine loading conditions. I can reproduce it reliably when letting off between 3,000-4,000 rpm or shifting back into 2nd at relatively low rpm (when slowing down over a speed bump on my road I push in the clutch to coast over it then re-engage second at a somewhat low rpm). It squeals very briefly (no more than half a second) and goes away. It's just a quick, sharp sort of squeal. My first thought was the belt or something it's driving. The belt only had 28k miles on it but was replaced way back in 2012, so I started by replacing it and giving all the pulleys a turn while I was in there to check for play, squeaking, etc. The belt I took off looked just fine and I didn't find any obvious issues with the pulleys. I put the new belt on anyway, but sure enough it didn't fix the issue. I replaced the water pump four years ago preventatively, and it only has 4,000 miles on it. I don't have any service records indicating the alternator has ever been replaced for what it's worth (I've owned the car since 2016 and the previous owner had pretty good records, so I suspect it's original). The car has spent most of its time garaged, coming out for summer driving, in case that factors into what the noise might be. Unfortunately, since the noise only shows up in specific conditions, I can't just pop open the hood with the engine running and listen. Well, I can—and of course I did—but everything sounds good to my ear. Any tips on what to check next? It's clearly related to specific rpm's and load. The only other thing I can think of is the starter. The squeal actually reminds me a lot of the sound a failing starter makes that doesn't immediately disengage when the engine starts up. I had this on an older Boxster and the sounds seems very similar, but it's been a number of years so I won't swear by that. Would that even make sense? Is it possible for a starter to stick in certain conditions well after startup? Any tips would be greatly appreciated! Hope you all are doing well!

Check the vacuum level at the oil fill cap, you may have an AOS on the way out.

Interesting, thanks JFP. I hadn't considered that. The AOS, breather tube and vacuum reservoir were all replaced four years / 2,500 miles ago so I'm hoping they're still good, but I'll take a closer look. Appreciate the tip.

I realized this evening that I can reproduce the noise without the engine being under load. It happens _exactly_ at 3,000 rpm when just revving the engine in neutral. It happens as I cross 3k when accelerating, and happens again when I cross it decelerating. If I hold right at 3k rpm, I can get the squeal to just sit there and make sustained noise. I tried to capture it on video. If you listen closely, you'll here it a few times in this clip. For the last few seconds I have it held right at 3,000 rpm and it's making the noise constantly. It's actually pretty loud even though it's a little tricky to hear in this video over the general engine noise (easier I find with headphones). It's really hard to pinpoint, but I'm pretty sure it's not coming from anything attached to the belt. It sounds like it's actually coming more from the front of the engine on the driver's side, but I could be mistaken. Is it possible this is caused by some variable valve timing or variable intake manifold system? Is there anything that switches over around 3,000 rpm on these engines that might be causing this? Any other thoughts on things to check? Thanks again!

Ugh... and it’s definitely not related to the belt or one of the pulleys it drives. Took the belt off and gave it another quick check, and sure enough, the noise remains with the belt off. This makes it instantly seem more expensive, but we’ll see. So... thoughts?