This Noise Is Driving Me Crazy. Please Advise.

[1987 GMC Jimmy]

Hey, guys. I wanted to as about some noise my car's making. Here's how it goes. Since I got it, it'll do this little tick only in gear driving down the road, and its speed is dependent on the engine speed. Well, it's gotten much worse. I start the car, and it sounds fine. I then put it in gear, and the noise starts. It's just a steady tapping that's loudest at the front passenger side area. There's probably one tap per revolution. The car had been running for a few minutes, and then I shut it off. It's been dieseling since I got it, and I have not been able to cure it with different idle mixtures, timing settings, etc. The engine temp is about 210 so about fifteen degrees more than nominal. When I shut it off tonight, it dieseled and blew smoke up through the carb. I tried to restart it, and it started up quickly, but it ran really rough and started making a loud clacking noise that varied in loudness and intensity. Almost immediately the car idled up and it helped the sound. I'm assuming the knock sensor picked it up and "knew" to do something about it. This is about how it's been going. I took it to the hardware store a few days ago, and it started up and ran there fine. When I started it up and came back, it was making the clacking noise in gear at low RPM. As soon as I gave it a little throttle, it went away, and it stopped by the time I got to the house. I pulled the valve covers and inspected the rockers and pushrods. There's nothing to report; everything checked out. It's not as aggressive sounding as bearing or rod failure, and the sound is not projected through the tailpipe, but it's plenty worrisome. I will note that the RPMs fluctuate up and down whenever it sounded its worst, and the engine started chugging and trying to die. I'm thinking that this is some kind of top end issue, which is surprising because this motor only has 60,000 miles since factory reman by Jasper. Any help would be greatly appreciated. I tried to post a video, but my internet isn't that great here in the sticks.

 

[moorevisual]

excuse me for the stupid question if it is one, but could it be an exhaust leak?

 

[74 Shortbed]

From the description does sound like a header leak??..

 

[Georgeb]

Throwing this out there but what about the exhaust crossover for the intake.

 

[1987 GMC Jimmy]

I'm not sure. I haven't even been home to play with it in a little bit. I will say that when you shut it off, it'll puff a stream of smoke upwards originating from around where the manifold bolts on. It's just so damn cluttered around there that I can never trace it with any degree of certainty. The thing that really worries me is when I start it up after it diesels. It's a much louder "clack" type sound. Let it cool off and rest a while, and it'll be fine. I think part of it is some type of exhaust leak on or near the engine, but I'm worried there's something else. My timing's a smidge advanced so I'll retard it a couple degrees and see what happens.