1981 240D Strange knocking sound
 

I have a mistery that has got me bumfuzzled! This has happen to me on at least three ocasions. While driving on an extended trip at least two hours or moremybenz's starts a knocking sound that sounds like it is comming fromunder the read of the car.At first I thoughtthat the muffleror exaust was loose but I ckecked it and it was tight.It get pretty loud at times and I pulled over to check it out but couldn't fine anything.If I let the car sit for awhilesay 10-15 min. it's finebut after awhile of driving it starts again.While driving in town this never happens only on extended trips.Any advicewould be appreciated.