Noisy plexi build

[jack2911guitarblues]

Hi everyone!
Any help would be much welcome. I built a Plexi clone a while back, very satisfied how it turned out. Recently I modded it by putting in a switchable Fender tone stack (including different mid & treble caps)
Worked very nicely, so I decided to install the mod permanently. At first all good! Now there is a loud constant “rustling” sound like wind or water at the beach. Disconnected the mod, no difference. Resoldered all components on the board, checked all resistors & caps but still same noise. Changed 3 preamp tubes, no difference.
Could it be perhaps coupling caps faulty?  Or more likely sloppy solder work?
Thanks Guys.

[Auke Jolman]

Posting some hi-res pictures can be helpful