Gap in roof liner normal? (pic)

[InfinitiMe]

I assume this is normal, but most of the length of my roof liner along the rear windshield has a gap i can easily put my fingers in. I'm not pulling this down even a hair; I can just put my fingers right in.

Is it normal?

I have a subwoofer in the trunk and that gap is rattling on certain frequencies of bass.

I'm thinking of filling the length and as much width as I can of that gap with Dynamat or something.

[palincal]

Yup, it is normal.