Chattering noise when shifted into reverse

I'm looking for some help diagnosing my problem. First, it is a 1990, Ram 50, 2WD, 2.4 liter 4cyl automatic with 19,000 miles. I get a chattering noise when the truck sits overnight or longer and I start it up, let it idle briefly, then move the selector lever into reverse. It chatters for a second or 2 then never again on start up until it sits at least overnight. In normal use it never chatters unless it sits overnight or longer without being started. Once the short chatter is over it drives perfectly.