Knocking noise at start-up

I have a 96 GP with a 3100 and about 81000 miles on it. When I start it cold it makes a knocking sound. It will clear up after a few minutes, but always happens after the car has sat overnight or for several hours. It doesn't even have to be that cold out, as today it was around 45 degrees where I live. Anything to be alarmed about, and what can I do to shut it up????????????Maybe I'm the only one that has this problem. Little help fellow gp owners.