Chippings in underbody protective covering

I have been hearing rattling noises coming from under the car over the past while, like tin cans rattling and it has become more noticeable recently.  I checked underneath today and everything seemed okay except for a gap that I found between the plastic underbody cover and the chassis.  BTW I'm referring to the normal protective cover either side of the exhaust and not the rough road package.

At the edges of the cover (closest to the door sills) there is small gap which forms a shelf and I found huge amounts of road debris and chippings along the inside of the cover.  I eventually removed it by running my hand along the gap but it took a long time!

Anyway some of you may have come across this before (or maybe my cover should fit better) but I just said I would point this out in case any of you experience this noise and it may rule out one reason.

Don't have photos as access to the area is awkward for obvious reasons but I'll try and get some.