"Mystery Noise" in ' 99 KW W900L

Okay... Perhaps you fine experts can help me solve an ongoing dispute I have with my other half. As well as solve an actual problem with the truck! (Not sure at this point which is more important)
There has been this mysterious noise for about the last six months or better. While seated in the cab driving, we can hear it in the bunk area. It almost sounds as though something is "swinging and hitting". For example, at first I thought perhaps it was a loose seat belt strap with the buckle hitting something under the bunk. As this mystery noise is in sync with the bumps of the road etc.
I checked that out... it's not the problem. Then I thought maybe it was a hanger in the closet swinging and hitting the wall. Nope. Not that either. I've checked everything within the bunk and found nothing.
Then one day, I realized (which HE vehemently argues against) that the sound was actually coming from outside of the truck. I thought maybe something in the side box... Nope. Maybe air hoses swinging and hitting back of truck.....Nope. I SWEAR i'd bet my last dollar that this noise is not coming from inside of this truck.
Recently, I decided I must get to the bottom of this annoyance. I don't drive so I can't send him back to the bunk to investigate for himself. I placed my ear on the floor of the bunk and could DEFINITELY tell that it was coming from outside of the truck, most likely near the rear underside. Again, checked it out. No loose fixtures, hoses etc.
WHAT ON EARTH COULD IT POSSIBLY BE? My very last guess is the shock on the driver side. Not an educated guess but I noticed that the "rail" that the shock is attached to runs along where I can hear the noise on the inside when I place ear to floor. Also, it's more pronounced in the rear close to where the shock is attached.
Anyone have any ideas of what this could be? Bender? Buehler? Anyone? Bender?

Thanks in advance. I can't wait to figure it out and FIX it! But even more so... I can't wait to prove I'm right.

~BridgettAnn

 

Oh goody then... I'm probably right! hmm.. now, to get to the bottom of the noise. I will check these

things ASAP and report back. But first I have to figure out a way to present the ideas of what it could be without him knowing "another man" told me. Sometimes the ego can make one one's own worst enemy.
Thanks, you guys, for this very helpful information.
~BridgettAnn

 

Okay... of course I'll be the one jumping up and down in the sleeper. Anything to prove my case.

~Bridgett