Dear amicloud,
I have same experiance of phenomenon. Of course, I don't know the fabrication situation; structure of filter. I guess you apply inductive iris filter.
The problem is the flatness of surface in body and cover.
Small gap exists in connect plane. And if you fabricate it by CNC, body is twist, maybe.
You must check it.
I suggest method.
1. grinding.
2. solder cream including silver
3. more many screw
If you share structure, I can check it more detail.
Thank you.