Loud Noises : 1958 VW Pickup

Hello Forum Members:

I have an early VW Pickup (36 H motor). I have been trying to get through this Summer without having to replace the motor. The cylinders were recently checked for compression and all cylinders have a reading of 90 PSI.
The truck had a complete tune-up by air-cooled VW specialists this summer as well. Here's the story...

Yesterday it was 90+ degrees, I had to drive my truck across town. I drove about fifty miles total. Everything was cool until I pulled in my neighborhood in the evening and started to hear a wrapping sound (like a hammer knocking). Sounded as if the noise was coming from under the truck, but not from the engine compartment. Then I shut down for the night. I checked the oil and I was running about a half quart low. I test drove the Pickup again this morning, the knocking does not sound like it is coming from the engine compartment. I hit a bump today and the knocking sound started again. The knocking is not constant, just at slower speeds. I revved the engine at a stand still, but no knocking.

Is there anyway I can do a self test on the motor? I want to rule out a bad rod in the motor. Could the tranny be making this sound?

Please help !!

Thanks,

Casey