I just went through this on my 2001 3.0 with 126000 miles. Tons of info online about this problem if you Google it. Mine was #4 cylinder, and it was a bad exhaust valve seat. This is a too common problem with these heads. If you don't hear an obvious valve tap from your cyl 5 while it's running, then most likely it's the valve seat. Valve tap would indicate a broken or weak valve spring.
To diagnose, I removed the valve cover, and rotated the engine until both valves were completely closed on the bad cylinder. I took the hose from my compression tester and screwed it into the sparkplug hole. Hooked an air line with low pressure (10lbs) to it through the quick connect fitting, and listened for the hissing which was heard at the exhaust tailpipe. This indicated that the exhaust valve was not fully seated. Hissing from the intake manifold indicates a bad intake valve seat.
After removing both heads, I turned them upside down and poured water over the valve heads to see if they would hold back water. 4 out of 6 exhaust valve seats were leaking. #4 ex valve let the water escape at the fastest rate. All intake valves had a tight seal.
I bought rebuilt heads from ebay for $350 for a set of 2. They came with new head bolts and head gaskets. They're working great so far. PM me if you'd like the seller's information. Hope this helps.
