Remington used to manufacture it own stocks and forends for it's rifles and shotguns. No longer is this true. It could be that the forend has shrunk causing it's grip on the forward reciever to loosen. Walnut (or any wood) will do that over time if it was not properly cured and dried when first milled. My 12 ga 11-87 that I've had since 1994 has a satin walnut stock and forend and has not ever had this problem, still fits as tight as the day I bought her. A rifle is different though, you don't want the forend touching the barrel of a rifle, accuracy will deminish and that could be one reason for the loose fit of your 7600. Just know that Browning has also had this same rattle problem with some of it's Gold series shotguns as well.

remington also used twin rails on there forearms which is a hell of alot quieter then the ol single rails from yrs ago. i had one in 270 and it was about the same as your saying but i didnt worry about it, just kept huntin