Chev truck engine noise on start

1999 Chev C2500 with 350 cid. When cold, starts quietly, but, when warm makes a deep clunk, thunk sound a couple of times like maybe a bearing knock. Drives quietly and normally. Doing this since about 40,000 miles, now 62,000 and no worse. Oil & filter changed ea 2500 miles, Pennzoil or Quaker State 10W30. Hardly ever carries any load at all & treated like a baby. If anyone has information on isolating this problem (if it really is a problem) I'd sure appreciate comments. Thanks.