Hi b0jang.

I would expect a loudish sound if the volume is maxed out. I only use a battery powered amp for testing, but it is very audible on a small amp volume.

It is good that the switch and pups work appropriately. I would not think that it is the shielding not being grounded (since it would be noisy! Or if a hot wire was grounded on the shielding there would be no sound). It could be a poor solder (or dry joint).

Can you include a close-up photo of the wiring in the cavity? That will make it a bit easier to problem solve.