[1976gmc20]

By "sleeper" do you mean a topper/shell over the entire bed, or one of those enclosed things that just has room for one person to lie down right behind the cab ???

If you mean a topper/shell then Yes there will be a lot of noise unless you fit out the bed with a floor or carpet or some beds, etc. We had one when I was a kid and until we got the interior installed there were all kinds of weird noises including cracks and pops from the suspension.

We had the kind of "boot" where you take out the back window of cab and front window of camper and have a vinyl sleeve that snaps into both sides. Works pretty good except it is more or less permanent since you don't have a back cab window if you take off the camper. Finally got rid of the boot when we got a new pickup and just had a window put into the front of the camper. A pickup cab heater doesn't seem to keep up with all that space.

I never had the kind where you just have matching sliding windows and a "doughnut" that seals between them but it seems like a good idea if you want to take the camper off sometimes. But you would have to be pretty slender and agile to get through it.