When You are Shielding Conductors,
The Shield is thought of as a Drain.
It is only connected on one side,
normally the Power Supply Side.
If the Shield is Drained on the Load Side & It took a Spike, that would Drain back inside of the Shield, thur the Grounded Conductor & have to travel Twice as Far !
If the Shield were to be connected on both ends, It could Carry Current.
This Current could be Dirty.
If this happened, You could Cause the Problem, of Noise Induction, that You were Trying to Protect against !!!
I could be Wrong ??? So Please tell Me
But This My Understanding of what is going on.
Except, I don't think this is Your Problem.
When I was a kid, I could use a small transistor radio, tuned off station, to Find souces of Noise.
I would look around the Backlight Invertor.
Good Luck ! * StarFish *