347 Stroker noise

[Gustav]

Hello to the masses! First post and it's a good one. I've got a 347 stroker. I really can't tell you all the specs since I bought the car with the engine already in it. Here is the problem. After running the car for about 20 mins I start hearing a noise that sounds like a valve out of adjustment. Rear cylinder, passenger side of the engine. But the noise is down around the camshaft area. Lucky for me I could take the valve cover off without removing the intake. Started the car and checked the rocker arm to see if it was loose and it is not.
Anyone out there know if there is any common problem I should be looking for. I am starting to think it might be a wrist pin or piston skirt. In my experience, lifters tend to make noise when cold and get quiet after the motor gets hot. This is the exact opposite. And I mean this thing gets LOUD after it gets hot. I know it's not rod bearing knock. Any help would be GREATLY appreciated.