2000 sierra 5.3l knock

I just purchased this truck a couple of months ago. I bought it with about 58000 miles, and have put on about 1200 miles since. I am now noticing a knock/tick. It seems most evident when the engine is warm. It is not noticable while the engine is under load, but as the engine "coasts", the rattle is quite prevalent. The noise seems to be from the top end of the engine. Help!! Did I buy a lemon? Is there an economical solution?

Usually this noise is caused by a split oil pump pick up tube O ring.
Replace the O ring and that should take care of it.