Ford Repair: Diagnosing Ticking Sound, exhaust leaks, aircraft mechanic

Question
Hello, I own a 1996 Ford Explorer with the 4.0L 6 cylinder engine.  The vehicle has 158,000 miles.  I change the oil EVERY 3000 miles.  Never had any engine or transmission problems.  Normally my wife drives this vehicle.  

Today I noticed an irregular ticking or clicking sound coming from the engine area, irregular being a key word hear.  When the vehicle is in park or neutral there is no click or tick.  While in drive you hear the tick.  I set the brake, put the vehicle in Drive and climbed all over it with a 5 ft piece of rubber hose to my ear as I put the other end all over the motor in suspect areas.  I couldn't find any exhaust leaks.  When I removed the oil filler and put my hearing tube into the hole the I could hear the sound, but it wasn't so obvious that it was a valve noise necessarily.  I believe it sounds a little louder when I am under the vehicle, then above.  I'm trying to figure out the best way to determine if I have a connecting rod problem or a valve problem.  In other words, head job or Long Block.  Any suggestions.  If it helps any I am a certified as an A&P aircraft mechanic.

Answer
if it is a rod you can disable one cylinder at a time and when you reach the right cylinder the noise will change or go away because the lack of explosion in that cylinder should take force off of that rod otherwise without listening to it i don't know. good luck