B6 S4 whining/resonance at 2500-3000 RPM

Hi all, I have been trying to diagnose an odd noise that has appeared on my car. I replaced the shortblock a while back due to the classic cylinder wall scoring/oil consumption issues, and had the heads rebuilt at the same time. I actually did all of those things TWICE but that’s a separate story…

Anyway, not immediately after the rebuild, but about 7,000 miles later I started to hear a strange sound from the engine. I would describe it as a “wooo,” “hoot,” or almost a “ringing” sound rather than a whining, grinding, or hissing. It begins to appear around 2500 RPM, peaks in volume at 2900 RPM and almost completely drops away to nothing at 3000 all the way to redline, in all gears and when parked in neutral. Using a stethoscope I was able to isolate it to the very back of the RH valve cover/cylinder head area. The sound is significantly more pronounced when the car is cold and fades somewhat when warm but doesn’t entirely go away.

This has me perplexed for a couple of reasons. First of all, most rotating components with some malfunction causing a whine should increase in pitch throughout the rev range, but this almost entirely stops at 3k onward. Second, there really isn’t anything IN the rear of the RH head other than the timing chain adjuster. Of note, I replaced both adjusters with the improved JHM units during the rebuild, along with new chains, tensioners, guides, seals, etc, full timing job. Everything went fine with the timing, and logging with VCDS shows the adjusters phasing as expected and within the same realm from LH to RH bank.

I suspected that perhaps the PCV was involved with this noise since it’s known for failing and making noise, and one of the pickup tubes is in that area of the valve cover. In fact, it was making some noise of its own, which went away when I replaced it (side note, I’ve replaced that valve like 4 times over the course of owning the car, usually as a “let’s try this” tactic since it is cheap), but that didn’t have any impact on the original noise.

I also suspected that perhaps the SAI could be involved, since one of the combi valves is in that area, the SAI is known for its problems and the noise is louder when the car is cold, but from what I’ve seen, the noise the SAI makes is different, comes from the pump itself at the front of the car, and only lasts for less than a minute whereas my sound never really goes fully away.

The only other thing I can think of is just natural resonance/harmonics happening at that RPM. I do have JHM headers and FI catback so there are some different exhaust resonances going on, but I would expect those to sound more like a buzzing noise - also, as I stated, the sound didn’t immediately appear after my rebuild/exhaust upgrades.

I’ve been around the block with power steering issues on this car and that sound is definitely different than this. I’ve also dealt with a failing alternator (had to upgrade to the Hitachi one) which produced its own interesting noises, but again not the same. I have also replaced the alternator belt, idler and tensioner which is often the culprit for whining noises in this RPM range, but that is definitely at the opposite end of the engine from my sound.

I do not have a video of the sound but I suspect I will need to record one to get some help on this topic, so I will try to do that and add it here.

I hope someone has some ideas because I have run out of them.