squeaky suspension

I notice that when im driving around town and the truck hits suddenly mild rough surfaces the there is a squeaky sound that sounds like comes from the front driverside tire however when the passenger side tire hits sudden hard surfaces the sound comes back again:banghead: its really annoying anyone got an idea on what it can be?????

I've noticed lots of silvy's do this once they hit about 4 years old. Not sure what it is, ours were all beat up farm trucks, so we didn't care as long as they ran.