The 7600 is basically a shotgun action. There will be some movement in the fore end. I have an older (1954) model 760 that had the same issue with noise. Most of the rattle was coming from the action bars impacting the grooves they slide through on the receiver.
My solution was to put plastic 'spacers' in those grooves to eliminate the extra room the action bars were moving around in. I made them up out of the cover of a notebook binder. May not be real high tech, but it worked.
Another area of loose fit is clearance between the action tube and the shaft it rides on. I used my high tech tactical low friction polymer notebook cover as a spacer in there too. These modifications were easily accomplished in an afternoon. The rifle is very quiet now (until the trigger is pulled...) and I am very happy with it.
I would assume that a brand new rifle would not be as worn as mine was and would fit together tighter and quieter. I am also suprised Remington has not addressed this over the last 50 years..... It may be that the example you handled was loose due to tolerances adding up, but that shouldn't make a big difference. I recommend that you look at another if you can - these are great rifles - and if they are too noisy things can be done to correct that. The intended use of the rifle also needs to be considered when deciding how much, if any, noise is acceptable.
