Bora WTO rim brake “groaning” noise — toe-in not the issue, what gives?

[Xrslug]

Hoping for some help with a braking issue on my Bora WTO rim brake wheels. Bought these used in excellent shape. Mounted and using with Campy red brake pads. Front braking is good, strong braking and just the normal “zzzzz” noise of the textured braking surface when braking.

Issue is with the rear wheel: under braking it has an obnoxiously loud honking or groaning noise from what I assume is brake pad vibration or slipping/gripping. This isn’t the the high pitched shriek that can happen when carbon rim brake wheels heat up. Toe in on the pads makes it *worse* - it is actually improved slightly by eliminating toe in on the pads. So in summary:

1. Honking noise is only from the rear wheel. Front wheel has no abnormal braking noise using a slight toe in on the pads.
2. Noise from the rear wheel is worse when the pads are toed in and better (but not eliminated) when pads are flat to the rim
3. I cleaned the braking surface with isopropyl alcohol before riding and I’m using Campy red pads
4. It’s not heat related — same noise when braking on cold wheels
5. Wheel is visually true and I feel no pulsing through the lever while braking
6. No noise under very light braking (just enough to have the pads rubbing slightly) or when braking hard enough to skid the rear tire

Any suggestions? The only thing I can think to try at this point is a reverse toe in of the pads (I guess a “heel in”). What’s throwing me off is that it’s only the rear wheel, which suggests to me that something other than the brake pad / rim interface might be causing this. Wheel alignment in the frame? Wheel slightly out of true (even though lateral trueness looks perfect visually)? On the other hand, I’ve had no issues with the alloy wheels I’ve used in this frame (which would point back to the carbon brake track / pad interface as the issue). I’ve searched for this specific problem and find nothing other than toeing in the pads, which is not solving my problem.