Over bumps I get a noise when the sheet metal rattles. There is a small gap between the skin and the support structure fo the trunk lid. It rattles & I thought about squirting foam insulation in there (it's not accessable close to the holes in the structure) but I am worried about distorting the skin when the foam expands.

Anybody have any experience with this problem?

Had the same problem. Took an old rusted deck i had and did the foam thing,warped the deck! Tried on other side (very little foam) didn't warp . The key is to use very small squirts,very hard to controll. You could also use cardboard cut up in small squares and squeeze it in between the supports and it will stop the rattle and it will have cushion.

us a piece of foam instead of the cardboard or foam insulation, it will cushion the two pieces of metal with out a problem. The cardboard will eventualy rub thru the paint and cause rust. the piece of foam wil work the best.

The problem is, the area causing the noise isn't near the access holes. Mayby a stick with foam stuck on the end?

By all means if you do this use something that is intended to do such a task. There are hundreds of products out there MADE to do this. 3M, duramix, fusor, sem etc all make a NVH foam.
A NVH foam is what the OEMs use as a deadner between body panels. Dont go to home depot and buy a can of expanding foam its NOT the same stuff.

My trunk lid made the same noise, and I applied some low-expansion spray foam (not the common 3x household foam) between the panels, and filled the major cavities. It is quick to apply and low-cost (under $10). I used care to not over-fill the areas. The foam has worked great for over five years. It even helps to reduce sheet-metal vibration from the trunk-mounted subwoofers.