Car - 1986 740 Turbo Wagon
Engine - B230 FT ( Mobile I synthetic starting at 75,000 miles)
Mileage - 235,000

After lending out the car for several days, it returned with an obvious engine knock/clanking. It is somewhat muted at idle, but when revved or put under load (hot or cold), the noise increases in volume and has something of a metallic tone. (The last time I heard something like this was when a spark plug worked loose on one of my cars.) I was hoping to find an exhaust leak of some type, but so far I've struck out.
I pulled spark plug wires out individually, and found that #4 cylinder seems to be the culprit. I've swapped plugs, spark plug wires, and fuel injector wires, but the noise stays with #4. Compression on all cylinders is good (160-165 psi), and valve clearances are perfect. Even with my stethoscope I cannot be sure of the source. (Note that my diagnostic hearing skills are pretty poor.)
Although I would like to avoid it, I guess my next step is to drop the pan and take a look. I know that with this mileage anything can happen, but what's bugging me is the suddenness of how this came on. It seems something would have had to break, rather than a progressive wear condition.
I was wondering if there is a more foolproof way of checking for an exhaust leak at the manifold. In this case, would adding something through the spark plug hole of #4 cyl and then starting the engine show anything ? Thoughts on any other diagnostic steps to try before tearing into the engine would be greatly appreciated.