Engine rattling/clacking noise
 

My '02 Dakota 4.7L V8 started making this noise much more pronounced about a week ago and has gotten progressively worse. You might be able to see a video I took of it at http://www.youtube.com/watch?v=6CIrnIMJe-I, it wouldn't load onto hear. The noise in the video is muffled or dulled, in person it sounds like a ball bearing being shook in a coffee can rhythmically. At first I only heard it upon accelerating (sounded normal in idle or just cruising) but now I hear it all the time and it just gets faster and a little sporadic when accelerating.
The vehicle feels like it's operating normally, I don't notice any actual operating problems when driving it but the noise is awful and has me thinking my engine is going to crack in half at any time.
Mechanic that heard it today says it was hard to be sure without removing parts but he thinks that the bolt attached to the rocker has loosened or broken off and that eventually the piston will break and I'll need a new header, says he's seen it before on my type of truck.
The sound is more noticeable on the passenger side, I did neglect to change my oil for 2 months longer than normal and I do have an oil leak so it got low just before the noise started being noticeable but I've changed it since and it's full plus I added some stop leak fluid to it, this hasn't had any effect on reducing the noise though. My check engine light was on until I changed the oil but is off now and the truck isn't giving me any other indications of it having problems other than the sound. Mileage is at 162k with only standard maintenance ever being done though sometimes delayed due to expense.
Is there any steps I can take to get a better idea of the problem without major part removal? Mechanic said I was looking at a few hundred to several thousand for complete diagnostic and repair, he seemed pretty swamped and rushed though and I'm afraid to drive it around anymore for multiple opinions.
Any insight would be appreciated, sorry for the long post but I wasn't sure how much was relevant.

i would pull the valve covers off and check rockers

Lifter??

If you can't tell which valve cover the noise is coming from, one of these will help if you don't want to take them both off:
http://www.harborfreight.com/mechani...ope-41966.html

Then remove that valve cover where the noise is coming from.

From what I've read, the 4.7's sometimes have a rocker arm fall off. This can be due to the lifter failing (plugging), or a loss in oil pressure, etc.

I bought an 02, w/ a 4.7 a couple weeks ago that ran bad. I thought that was what was wrong with it, because it had a rocker fall off. But it dropped a valve seat.

Once you have the valve cover off, if one of the rockers is off, check the height of its valve stem. If it is higher or lower, it may have dropped the seat. You can measure the height by putting a rubber band or o-ring on a drill bit, and use it as a depth gauge.