Birds chirping -- a familiar sound with my Explorer. As others have stated, I believe Ford only made the Explorer with Timing Chain motors; in any event, your 2000 most certainly has a timing chain. If the sound does indeed sound much like birds chirping: check the idler pulley (these go bad around 100K or so, cheap part to replace), check Tensioner / Tensioner pulley (also go bad around 100K, not too expensive) ... it could be other bearings, like alternator, etc. all of which are cheaper to fix than "internal engine problems.". Also, the Gates-made replacement belts that were tried on my 2002 both chirped like crazy (and they were the right size, tried 2)... ended up putting a Motorcraft (ford) belt on, it's all quiet now. I'd get a second opinion from another shop. If it were the chain (and if it's a SOHC 4.0, they were known for some chain tensioner issues until around 2002), you'd hear a rattling noise, often around 1500 to 2000 RPM. This can be expensive to fix if the tensioner that has failed is the rear one. Any ford mechanic could help make this judgement call for you. If your 4.0 Engine is the "OHV" version, I seriously doubt you'll ever have timing chain problems.

Oh, and as a previous poster said, if your engine is the OHV version, it could be the cam synchonizer... these do go bad -- but it's not a repair that requires a total teardown.