I think I can shed some light on your situation & it isn't GOOD. I have a 1997 4.3L that is doing the exact same thing. I did like you I put Seafoam in the intake, gas, & oil. I changed the oil/filter after 600 miles. But nothing changed. So myself & a couple of mechanic friends started to look deeper. I took a stethoscope & started checking. Well at first I thought maybe a main bearing. The noise was loudest at the pan. But being an old mechanic myself that didn't add up. So a little more research made it clear it is a wrist pin or piston. Mine doesn't start really rattling until a little over 3000 RPMs. I have a ticking after warm up that I hear & I believe could be a rocker. I'm going to drive it until it blows. I just took it to town & back 250 miles driving 70-75 MPH is ran good & got 20.5 MPG. I run 10-30 Valvoline synthetic & I have added some Lucas synthetic additive to the oil. When I change it I'm going to a heavier wieght oil. I'm facing an engine replace, but the price of these is dropping I plan to buy another one when this one blows & just keep it for parts. Of course I pay cash for my trucks so I have no payments & don't owe anything on my 97 Blazer 4x4. I just passed on a 1999 Blazer 4x4 Trailblazer Pkg. with 130,000 miles on it for $4500 asking price. They would of taken $4000 or less cash. But in a few months with the economy in the toilet there will be even better deals out there if you have cash.

 

Wow that is pretty discouraging to hear. Im in the same boat as you though im not going to pay a Mechanic 500 dollars to take a guess what it is i will wait to its worse and more ovious and then get it replaced. as needed.

But overall these 4.3L are great engines way better than my ford Explorer I discourage anyone from buying a ford explorer they are trouble waiting to happen.