b2600i noisy idle?

I just bought a 1990 B2600i with 241,000 miles. It drives great but at idle has a rattle like noise. The previous owner had the stock head rebuilt. I asked if he replaced the timing chain or if the owner before him replaced it. He said he didn't know about the previous owners, but he did not replace it and when they put the head back on the chain was tight.

The truck starts up right and if I pull on the throttle a little to give it a couple hundred more RPMs it sounds fine.

Any ideas? Could I check the timing chain by pulling the valve cover?

Thanks

I have the same problem with the daughters 1991 B26i. I would be very interested to see what others have to say. I have been told it is anything from a bad water pump to the chain.