I have a 2000 Ford Explorer V6 4.0, manual. I bought it brand new and have put just over 175,000 miles on it. I was driving down a dirt road to my home and noticed what sounded almost like a hissing whine coming from right in front of the driver seat. After about a mile the engine died on me, so I towed it home. Upon looking at for obvious signs, i pulled the driver side spark plugs out. The front two where fouled beyond belief, so I slapped a new set in and started it up. It now runs and idles fine, but as soon as you hit the accelerator it starts the whining again. I believe i pinpointed it to the head on that side, but before i tear it apart I wanted to get some ideas. Another fact i could throw in is since about 145,000 miles ive had a lifter tick, a common problem with ford explorers, but that sounds no different now.
A tiny exhaust leak can produce a whine, but prolly not foul the plugs.
One possibility is that your intake valve guides have worn to the point of letting it suck oil through the guides and into the fuel/air charge, thus fouling the plugs.
Concerning the whine, I have a 03 ranger with a 3.0l and when my valves wore out (the head tulipped) it gave a whine that became a whistle, but not at idle, just higher rpms under a load.
I think you are probably correct looking into the heads. Most likely need a valve job.
Do it soon before the residue going out the exhaust ruins your catalytic converter.